Here are my experiences.
Very important though. Only drop in fish that have been bought from a reputable seller. Otherwise, you need to follow that quarantine procedure that you can find elsewhere in the forum. Fish from stores like Petco have been well taken care of however, and are not a health risk to an Axolotl. I put those right in the tank and all is well.
I personally would quarantine any fish and so on for a good month, regardless of where they came from.
I would too, UNLESS you know specifically how those fish are kept, fed, and medicated. Petco (at least the one around here) is one of the stores that I know keeps all their fish species separate and under the right environments, feeds them appropriately, and keeps them medicated against all diseases. I suppose they have to, as they sell some of the fish (minnows, specifically) for the sole purpose of being instant feeder fish.
Sorry to burst your bubble, but I've worked at Petco and know how their systems are setup. The best case scenario is that one case (9 or so tanks) share all their water and have minimal mechanical filtration meaning that all fish are potentially sharing diseases. Worst case scenario the whole store's aquatic system is shared and all the fish are swapping water and microbes. Oftentimes with African clawed frogs, known asymptomatic carriers of Bd, chytrid fungus. Feeder goldfish almost always have their own setup with no shared water, which is good.
With regards to their feeder fish, the are shipped in, not quarantined but dropped in with the other feeders. They usually only last long enough to have a few dozen left by the time the next shipment comes in. Death rates are usually between 10-100 per day between the feeder gold fish and rosy minnows. I did dead counts every weekend for two years. And our store was known locally for having one of the best aquatics department. Petco fish should definitely be quarantined, but the sad fact is you are likely to lose most of your feeders.
I'd also advise against mollies as these are sub-tropical fish and do best at higher temperatures. Sustained temperatures below 78F can be fatal to them. Take a look at the Fish with Caudates article.
Hey i've actually kept tropical fish with my axolotl. Infact they get on very well with each other.
1 Pleco.
2 Clown louch.
I keep the tempture rougthly 20c - 23c and there all happy, The two clown louch are rougthly 3inches at the moment & the Pleco is 3iches also.
Just keep the tank large and they pritty much just stay away from each other and everything is fine. Its been like this for about 6months now.
20-23c is not a good idea for your axolotl and probably not warm enough for the other two. They may seem okay but sooner or later a disaster will happen, either with your fish or your axolotl will get unwell due to the temperature or the fish... Please separate your axolotl from your fish and put him into a colder tank. Basically, it is a stupid idea even if it has worked for 6 months. You will be surprised at how stressed your axolotl is right now. Please remove him...
Also, I hope you realize how large Pleco's grow to. It can be 67 cms and above. Your Pleco sooner or later will suck onto your axolotl and do a lot of damage. Not a good idea. Why risk it?