NO!
Never, ever mix amphibians. It makes me furious when ******* hole-in-the-wall pet store owners do this.
Amphibians have water-and-gas-permeable skin. They breathe and drink through their skin, and I wouldn't hesitate to say they their skin, as an organ, is one of the most complex in the animal kingdom (perhaps second only to the likes of cephalopods). Many amphibians have symbiotic bacteria that live in the outer layers of their skin, and some even have symbiotic algae that photosynthesize sugars when they are larvae. The Pacific Chorus Frog, Pseudacris regilla, actually has bacterial symbiotes that are capable of fending off the crippling disease known as chytridiomycosis, which is currently causing mass extirpations of amphibians in South America, and has even driven a few species to extinction; indeed, culturing this bacterium and attempting to grow it on susceptible species may be the only way they will survive in their now-contaminated former habitats.
And yes, many amphibians are capable of excreting extremely potent toxins from glands in their skin; true toads (and other anurans) have glands large enough to see just behind their eyes, called Parotid glands. The rough-skinned newt (Taricha granulosa) and the other members of the genus Taricha can excrete a toxin identical to the toxin that can be found in the japanese blowfish; tetrodotoxin. Tetrodotoxin has no known antidote, and if you should accidentally ingest it or absorb it through a wound, you will likely die. The newts don't even make the toxin themselves; they make it through a symbiotic relationship with another bacterium, unlike the infamous blowfish.
Amphibians tend to secrete this chemical when they are stressed, and amphibians are easily stressed by poor environmental factors, such as crowding and species mixing. It is not known why, exactly, amphibians dislike being kept in close proximity with other species of amphibians, but ask anyone with real experience keeping them and that's what they'll tell you. It is likely because amphibians are shy creatures that are instinctually aware that the presence of another creature likely indicates either competition for prey or an attempt at predation. It would probably be fine to keep two or three paddle-tail newts in something like a 20-gallon long tank (depending on the size of adults and any behavioral quirks or other species-specific traits), but you would need to do some in-depth research to find out specifics.
Now that I have (hopefully) illustrated why you are never supposed to species mix amphibians, the environment within the enclosure is the next most important step. Amphibians, unlike (most) reptiles, dislike heat. Or rather, heat makes it difficult for them to breathe. This isn't as true for some anurans, especially tropical treefrogs and true toads, but even tropical and desert-dwelling species need temperatures below 80 degrees, or they will overheat and die. Most caudate amphibians (newts, salamanders, and their kin) need temperatures in the high fifties to mid-sixties, and anything higher than that will eventually lead to an early death. 10 years might seem like a lot, but many amphibians can live to be 30 or even 40 or 50 years old; cryptobranchid salamanders like hellbenders and the Japanese giant salamander can outlive humans, reaching a maximum lifespan of around 100 years. The current record-holder (to my knowledge) amongst non-cryptobranchids was a fire salamander (Salamandra salamandra) kept in a zoo living to 55 years.
So, back to the temperature. Newts need water in the mid-sixties at most; this can be troublesome in the summertime in hot places, and sometimes aquarium coolers/chillers are needed to keep the water cold enough. Newts are also easier to keep with a filter, but filters produce heat too (via the motor) and they obviously need a day/night cycle, so they need a light. These two additions can make it difficult to hit the proper temperature zone, but there are alternatives. LED lights are more expensive than flourescents, but they last much longer (the LED strips won't need replacing for 5 years) than flourescents, and they produce very little heat, while also producing the proper wavelengths of light for plant growth. Plants are used by some people instead of a filter to help keep the water clean, which works, but only if you have lots and lots of plants, and if you've never kept live plants before this can be difficult. Using LED lights and keeping the tank away from other heat sources is usually enough to hold the temperature levels low enough for the newts. If you have a basement, it may be best to have the tank there; basements are insulated from temperature extremes by the soil around them, and so stay warmer in winter and cooler in summer, making them perfect places to set up newt or salamander tanks.
The last part of the tank that's important to get right is the way it is furnished. Aquarium rocks, especially the painted ones, are not a good idea. They can be accidentally ingested by the newt, which will be unable to pass the stone and die. On top of that, plain aquarium gravel is a poor substrate for growing plants, which are important for helping the newts feel secure and for clean water. With this in mind, most people opt for play-sand from hardware stores. It's safe if ingested, and quite cheap. The only downside is that it is often dusty and needs to be rinsed with tap water in a bucket or a tub before being put into the tank. Finally, newts, like many animals, prefer to have natural items as aquarium/paludarium furnishings as opposed to fake items. Mopani/manzanita wood and pieces of slate and other stones are the best tank furnishings, as they help the newts feel more at home. Another material that is popular are terra cotta flowerpots; these are manmade, but since they are made of clay and mimic stones, they are an acceptable addition. Another interesting use for them is using them to grow epiphytic plants (plants that grow on surfaces). Plants such as Anubias sp., Taxiphyllum sp., and Microsorum sp. and others will grow on the porous surface of terra cotta if they are anchored there with rubber bands or a bit of string.
As for setting it all up, you really need to get everything ready at least a month in advance of the newts. That means water, rocks, wood, plants, etc. are in and any equipment is up and running for one month prior to the animals being in the tank. This is important, because beneficial bacteria that are needed to break down the newt waste need time to really get established in the tank, and this can take up to and sometimes more than a month in large tanks. You can accelerate it by buying a small bottle of beneficial bacteria for use with aquatic turtles, which is the same thing as what you want to develop in your tank. Add it immediately after setting the tank up, and you can probably add some weekly until the newts are ready to go into the tank. As for picking plants, be careful that you know which species you are buying and how to grow them, how big they get, etc. Some plants are hardier than others, and so would be better choices for a newt tank, since the newts enjoy walking on/in and hiding amongst the plants. Another idea would be to use a piece of glass siliconed into the tank bottom as a divider between a terrestrial side and an aquatic side of the tank, thus creating a small area for the newts to completely haul out if they so desire. This also creates an opportunity to grow terrestrial plants, and species in the genera Epipremnum, Philodendron, and Spathiphyllum are good candidates. Another option would be to use rocks to build sand up higher (pile the sand on top of some large rocks) than the water to create a shore area, and then use some cork bark/rocks/manzanita wood to hold the sand back. Then you could make a 1:1:1 mix of sand, organic (perlite-free!) potting soil, and organic topsoil and spread a thin layer on top. This could then serve as a spot to grow semi-aquatic or terrestrial plants; java moss or other aquatic moss-like plants could be draped along the shore, where they will attach and grow further into the shore area, creating a more stable bank for the newts and making an attractive green carpet in the tank.
Whew. Hopefully, this helps. Amphibians are sometimes over-simplified by people when it comes to care; they are much more complicated than things like tarantulas and ball pythons, mainly because of their unique physiology, which tends to make them more delicate than some people realize. You could keep some paddle-tails (Pachytriton sp., by the way) in a more simplified setup without the plants, but you would only be able to keep one newt in this kind of setup due to the loss of the water-cleaning service provided by the plants. Going with a more complex planted setup is more expensive and more work to build properly, but they need virtually no cleaning (asopposed to frequent water changes in a more simple setup) is a great way to observe the newts in their natural habitat, and the newts will almost always live longer and be "happier" in these kinds of setups.
addle tail newts are fully aquatic so no need for a basking rock
