I don't know about bottling, but there's no reason not to use gelatin in the primary if you keg. However, you don't want to stir it. You'll end up stirring up the trub and defeat the purpose of the finings. There's another way to use gelatin without a secondary, though; I just add mine to the keg and rack on top of it. By the time it's carbonated and you pull off the first 1/2 pint, it'll be crystal clear.