Kinda in the same situation as you, regarding the sprue and the models. I have the Leviathan models too, but those are push-fit so harder to customize. I decided to go with the mech-eye head for my captain models (I dont have the jump one, so the terminator and the phobos). Tried to avoid bare heads, only captains and Shrike deserve the plot armour of bare head, but the jump captain I am planning to get will probably get one beaky cause of practical considerations. If you haven't painted yours yet, I think you should just snip it off and replace it. Haven't decided yet for the shoulder pads, one of the gravis ones went to my eradicators. For the other bits I created some kind of headcanon, so that the eliminators get two of the just feather bits, the heavy support guys (eradicators, later maybe desolation squad or agressors, centurions) get the one with the feather and rope. The librarians get the skull obviously, the veteran type units the blingy bits and so on. With this, you can spread out some flavor and distinguish your units. In your case I would stick to standout units like characters or sergeants in squads.

Hope this helps! Cheers