I have feeder fish in there. With fish it is generally not reccomended for two reasons, the axolotls will probably eat the fish or the fish will eat the axolotls feathery gills. I've been keeping a close eye on my feeder rosy barbs and they don't cause trouble with the axolotls, they just swim and be eaten.
Axolotls are not agressive, but they are predatory, and they will eat what they can catch. Generally they're best in a tank entirely dedicated to them and maybe a few feeder fish presuming they are parasite free and don't nip the lotls.