It doesn't really matter how long you wait. You need to make sure they are getting enough food, either by making sure some sinks to the bottom or getting them sinking shrimp pellets. They won't scavenge waste or anything, they'll just clean up food that gets left on the bottom. If you aren't overfeeding, there shouldn't be much of anything down there. They'll do fine any time you want to add them... you don't need to wait to make sure there is already food for them, because you don't want a bunch of food rotting down there.