I suggest a few zebra danios. They're fast enough to pick off all but the really speedy fry, and I've never had them do more than occasionally dart at a guppy, they don't bite fins. You'll still get a baby guppy or two that survive to adulthood now and then, but it won't be a problem, and you can always trade them to a LFS.
My adventures in keeping Indostomus crocodilus, the crocodile toothpick fish. AKA armored stickleback, paradox fish, and freshwater pipefish. Care is (to the best of my knowledge) exactly the same as Indostomus paradoxus.