Hi Zoe!
The ramshorn snails come in various colors (red, blue, brown..) - I have brown ones and red ones. They are small (have never seen any bigger than 1 cm) but pretty and skim the underside of the water surface ("upside-down floating") which is amusing to watch. I can't keep these in the tank for any time at all because they get eaten so quickly. I have bought them at pet shops as well as from other axolotl keepers from the German forum.
I have read contradictory info about apple snails. Apparently they are tropical snails, needing warmer water than what is in my coldwater axolotl tanks. Mine do fine, growing very rapidly and keeping everything quite clean. However, I have been told that at these temps, they will only live about a year instead of about 4 years. We've crossed the one year mark and they are still going strong. They have not laid eggs, though, so either I managed to have nothing but females, or the temps bother them after all? In any case, they are wonderful cleaners, become large quickly (mine are about 3 cm across), and are fast-moving and fun to watch. Their tails have glittery apricot-colored flecks, quite pretty.
Bladder snails are a plague to an (axolotl-less) aquarium. You can usually get them for free at most pet shops and from fish keepers. You are very likely to get them when buying water plants. They are also very, very small, and their shells are very soft. They usually disappear quite quickly from my axolotl tanks - but there are always a few teensy ones somewhere. They are terrific algae eaters.
I agree with Ray about platys. I have both guppies and platys and whereas some platy types are quite pretty, they are very strong swimmers and a few will always manage to escape from being eaten. They also actually like the water a tick warmer than the 16-18°C I have in my tanks, and so do not breed as readily as I would like. (On the other hand, their fry hides at the bottom of the tank, unlike guppy fry, which could mean that although the adults are breeding, I never see the fry because it is getting eaten immediately.)
Guppies are wonderful. I have various colorations of fantails because they are pretty. Their breeding prolificacy is legendary, of course. So, however, is the appetite of an axolotl. I finally had to set up a separate breeder tank for the fish after my axolotls were going through about 15 fish a day per tank.
It is unlikely that they will overrun an axolotl tank (for long).
Hope this helps,
-Eva