Pam- The axolotl and tiger salamander are related species. There are neotenic populations of tigers in the southwest; you may have been sold one of these instead of a true axolotl. Axolotls can also transform, but less readily.
Austin- I haven't kept axolotls, but I have kept sirens and neotenic mole salamanders. My experience is that these big sallies can be kept with small, fast, peaceful fish; my sirens live with a group of bluefin killifish, and they leave one another alone. Sallies will eat fish they can catch, but they have poor vision and only try to eat what's right in front of their nose. Healthy guppies should have no trouble staying out of the axolotl's reach; you might lose a sickly or sleeping fish now and again, but I doubt the axolotl could wipe out a whole colony.
Like Pam said, there can be issues with the fish nipping the salamanders, but I don't think guppies would be a problem in that regard. I had a Seminole killifish in with my sirens for a while who tried to eat the salamanders' gills, possibly because they looked like bloodworms.
Another issue is with small, slow, but dangerous fish. One of my coworkers decided to put a tiny freshwater puffer in with my neotenic mole salamander; the salamander ate the puffer, killing them both. Cories and other spiny fishes could pose similar problems.