Well lets see if I can help
Should it have many plants - That depends on what kind of tank you are setting up. Is it an aquatic setup or a semi-aquatic or terrestrial. Depending on the age of you newts, you will have to set up your tank accordingly.
Substrate - Well that depends on the setup also. For a terrestrial one, fine gravel would work just fine as its easy to find and cheap. If you are not into alot of cleaning then not having a substrate would be best as it would be easy to clean a bare bottom tank of left over food and animal waste.
Stones to breathe - Are you talking about air stones or just large rocks that stick out of the water so the newt can crawl out on? If you are talking about just large stones, you can, C.o's like to take a rest from swimming all day and like to rest at the surface with just their heads poking out.
Cleaning - This will depend on what you put in your tank. I have gravel in all of my tanks and I honestly dont do alot of cleaning of the gravel. What I like to do is take the newts out of the tank and put in a clean corner filter and a clean Fluval 2 filter in the water. Then I mix up all the gravel and let what floats about get caught by these filters. Once things settle down I put the newts back in. The glass is easy to clean, just get an aquarium scrubber pad from any pet store and clean the glass when you see some algae growth. If you clean anything from the tank that you are going to put back in it, I would use a mild antibacterial soap and make sure you rinse that stuff really good before you put it back in.
Size of tank - How many C.o's do you have? I personally wouldnt put more than 4 in a 10 gallon tank. But if you only have one, why not a 5 gallon? Small, easy to fill up and clean!
Fighting - I think fighting wont happen with C.o male and females. If anything they will get along quite well and might breed for you if the conditions are right. I wouldnt worry about fighting.
Hope this helps you out answering your questions
