We kept our 2 month old guppy fry with one khuli and 2 weather/dojo loaches(and you know how big those get) and haven't had a casuality. Khulie loaches mostly hide anyways and fry are so active I doubt he would catch them.
Most likely kuhlis wouldn't eat guppy fry except maybe right after the fry are born. When the fry are born they will sometimes lie on the bottom to catch their strength and thats when a lot of fish will get them. Once they are active its harder for other fish to catch them. Kuhlis aren't going to be chasing down fry as their main source of food.