You can keep axolotls with coldwater invertebrates and even some fish with the proviso that some or all will eventually be eaten. Some animals will be able to escape and hide and others can outbreed predation, especially as captive-bred axolotls which are regularly hand-fed become very "lazy" and rarely hunt outside snapping at things right next to their mouths, but you can't keep anything in an axolotl tank that you don't want getting hurt. They'll even bite each other, though they usually heal thanks to their regeneration abilities.

I keep river shrimp with mine and they get eaten occasionally but haven't been wiped out yet, they're a convenient free-range sustainable food to supplement their earthworms. Cloud mountain minnows and coldwater guppies may also work if you're comfortable feeding live fish. Anything tropical or subtropical or brackish is a total no as their water can't be safe for the axolotls.