Most healthy bettas can fight off whatever it is a live food could be carrying. I know OldFishLady gets tubberwares, fills them with oak leaves and such, and fills it with water to attract mosquitoes. To harvest, she nets them, rinse well, and then feed.... She feeds the larve and whatever other larve she finds in those bucket/tubberwares to her bettas and she says they are all fine.
If you wanted to, you could technically start up a colony of guppy fry to feed. Maybe put together a 10 gallon set up with 3 females and one male.... this way you know their general health and the general health of your fry that you feed.