You can place them in with the adult fishes, when they are about an inch
Or if they are big enough, so that they do not fit into the adult fishes' mouth.
I usually leave my molly fries separately, as they can be independent and could be fed separately. They can be happy without any bigger fishes chasing them around. If they are frightened, their growth would usually be affected.
I am talking about the ones in the 10 gallon. There were twenty in there, but they kept disappearing, one by one. I soon found out what happened. There was a castle decoration in there. I found the missing guppy frys in there....except they were dead. I don't why they're dead in there. Any way there's only 5 guppy fry left in the 10 gallon tank.