Other Tank Inmates, & Other Axolotls
Here are the best three words of advice regarding other tank inmates: Just say no.
Why? Well, let us suppose we would like to keep something with our axolotls, for instance, a fish, or another salamander or newt. The fish will invariably attempt to nibble on an axolotl's gills. After all, they're so attractive and feathery! The salamander or newt may try the same trick, particularly at feeding time. Let us suppose that our axolotl is bigger than the fish or salamander. It's quite likely we'll end up with one well fed axolotl! There is a simple rule that axolotls follow: if it moves and it's smaller than our axolotl, it'll end up in our axolotl's stomach. So, follow my three words of advice: Just say no!
Young axolotls, less than 8 cm in length (3 inches), shouldn't be kept together in a confined space. If they are to be kept together, it would be wise to use an aquarium that allows them plenty of space. Young axolotls will nip each other's feet and gills, more so when very young. Even axolotls up to 15 or 16 cm (6 inches) may nip their tank mates, occasionally inflicting serious damage. This is particularly noticeable in wild type axolotls, since they are naturally more aggressive than homozygous colour mutants, and wild types will bite colour mutants in preference to other wild types. This behaviour is frequently observed by scientists and hobbyists alike and has yet to be fully explained.
Cannibalistic tendencies are much reduced in adult axolotls. However adults will predate upon considerably smaller axolotls, so keep different generations apart until they all reach adulthood.
Lastly, consider keeping the sexes apart, unless you are prepared to deal with the occasional batch of eggs.