How old do your guppy fry have to be in order to put them in the main tank?

Your guppy fry don't have to be one size, they have to be big enough so that other fish won't eat them. If you have any small fully grown fish then I think your guppy fry should be ok if they're that size. If you see any fish trying to eat them put them back in the breeding tank until they're a bit bigger.