Yeah theres lots of info about guppies on here, why dont you start up a breeding tank? only need one male and three females and your axie will have a constant supply! ive got a 60ltr on the go at the moment should get babies very soon! il just throw a few in for gomez evey now and then! as has been said already 30 days minimum quarantine for anything going into your tank so you may need to get another small tank anyway and if you have males and females within those 30days you will get babies! Ive found having a number of guppies in with my axolotl was just producing too much waste my lovely freshly vacuumed sand always looked dirty when there were 5+ guppys in there even in my big tank (hence why I moved them) also if you breed your own you know for sure that they havnt got parasites and diseases so no waiting for quarrentine!
I also found that the male guppys can get quite territorial even with an axie, they never touched gomez (they wouldnt dare get that close!) but they would behave very boisterously in front of him sometimes making him hide from them! Ultimately tho a guppy will end up in your axies belly tis just a matter of time!
Have you considered white cloud mountain minnows they are beautiful little fish so delicate looking (they also have some awesome shoaling behaviour when in a group, dont go near your axie, are quite quick so better survival rate than guppies, its awesome to watch them feed darting up to the surface one by one, and they produce very little waste especially when compared to a guppy so lots easier to keep clean (belive me it makes a huge difference!) i love em, theyre quick too gomez has managed to catch one within a week (he caught 5 guppys in one night!) so should be around for a while!
I just wish I discovered these mountain minnows before I decided to set up my guppy tank the lack of fish poo is a godsend! (dunno how easy they are to breed compared to guppys though!)