I'd try to get nightcrawlers as a staple feeder.
My axolotl has eaten guppies, but they are never intended as anything more than a treat. White cloud are a good choice for tankmates, but the problem with small fish is that you have a lot of matter on them that isn't necessarily beneficial to the axolotl and will just be pooped out and unused - leaving them having expended a lot of energy to digest something that wasn't very useful to them.
I breed the guppies myself and only introduce 1/2 size ones (between the babies and adults in size) - bigger fish can cause impaction. White cloud aren't too difficult to breed, but a bit more so than guppies. Guppies also tend to prefer warmer water - so when I move them from their normally 80F breeder to the 67F axolotl tank, their metabolism slows down and they tend to grow more slowly - allowing them to remain a safely eat-able size for longer than a fish that metabolises well in colder water.
Just an idea. I don't think they are worth much as feeders, they're just fun to watch, don't bother the axolotl, and won't hurt it when eaten. And it offsets overcrowding in the breeder - everyone's happy !
Feeder shrimp are also nice if you like having something else live in the tank. Again - they are a treat, not a complete diet.
Nightcrawlers are great and provide a lot of nutrition without much that is difficult to digest since they're soft-bodied and don't have hard shells like some other types of worm (which can cause impaction).
Hikari sinking massivore pellets are also great for them, are are trout / salmon pellets.
