Zoanthids sound like they meet all your criteria. No target feeding, hardy, easy to care for, great colors. That's where I would start, just make sure you place them in a place where they can spread over the rock decently and won't interrupt any future coral placement. They will grow almost anywhere so if you have some better placement for more demanding corals maybe leave those spots open if you plan to add more in the future.
I think you answered your own question.
