I always put the decals on the paint, and then clearcoat over both the paint and decals. That way both the paint and decals are protected. Allow your decals plenty of drying time, at least 24 hours, and you should have no problem. I've never had any decal problems this way. If you want to be sure, test a spare decal first. Apply it to a piece of spare plastic, let it dry, then clearcoat it.
I might also add that putting the decals under the clearcoat will help prevent cracking and peeling over time. The best thing to do with any primer/paint/decal/clearcoat combo is to test first.