I would keep all different exercises! Incline bar, incline db, flat bench flys, flat bb , flat db, cable flys low to high , for upper chest
All sorts of stuff
Decline really I would stay away from till u develop a better chest because decline is mainly used to tone the bottom of the chest after its developed !
True ectomorphs really need to focus on the top of the chest we tent to have a boney upper chest and that's the hard part to build
I did not start decline work till after my first two years of steady training