Yes u can paint over PC, the only thing you have to do is to sand slightly with 600 grit wet sand paper, mask then paint and clear coat.

As for the decals u mean water slide decals right??, well it depends a lot of the decal paper u are using to start with, to apply the decals u have to sand a tiny bit with like 2000 grit sand paper (thats how i do it ok? many other ways to do the same), clean really well with alcohol, put the decal in warm water with some soap in to, slide the thing in position, take bubbles out using a squeegee thingy, let it dry and then clear coat it with car paint clear or maybe u-pol #1 clearcoat that is car clear coat in a can, this wont get yellow with the time as hobby type of clear.