When dealing with PE pay close attention to the instructions. You may have to shave off detail to add PE detail pieces, or you may simply add a PE piece to existing styrene, or even throw out the original styrene piece and replace it completely with PE.
As far as painting is concerned, J-Hulk is dead-on. Assemble and then paint. Oh, yeah: The best glue to use for PE is cyanoacrylate (super glue).
Hope this helps.
Gip Winecoff