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Ghost shrimp aren't as completely harmless as people think they are.
I kept a few in a grow-out tank of N. multifasciatus; got the shock of my life one day when I watched a shrimp grab a young multi as it swam by! Needless to say, I pulled the shrimp and have never kept them with fry again.
i currently have a 3 gal desktop @ work, i have 1 trio of guppys with about 3 different generations of fry with them, i have 7 ghost shrimp in there with them & havent seen any problems.
You should be ok, if you are lucky the ghost shrimp will breed and your baby guppies will dine on shrimp snax. One thing to watch for is that many kinds of shrimp are sold under the label of "ghost shrimp". Some are species that get large and predatory. You should easily recognize them as a threat once they get to that kind of size.