Speaking as a fish keeper, guppies are NOT good live foor for anything. Feeder guppies are kept in underfiltered, overcrowded tanks. They come from fancy guppy stock, and are therefore HIGHLY inbred, which makes their immune system very fragile. This combines to create a perfect environment for diseases, which are easily transfered to other animals.
If you want to feed your axolotls (or any other aquatic animal) live food, I would suggest either rosy red minnows or ghost shrimp. These are just as easy to obtain, just a cheap, and have a much lower chance of being diseased before endtering your tank. Rosys should be perfect. They don't nip at other animals and are extremely resistant to infections and parasites. They should be a good size for adult axolotls, but depending on size, might be too big for smaller axies.
One other note. If you get a rosy that is too large for your axies to eat, get rid of it! theres a reason these are called minnows. An adult rosy can be over 2 feet long, so you don't want to keep these guys.