x2. Your birds are still young and spring is in the air. Their hormones are changing with age and with the lengthening daylight hours.They are getting bigger....it's spring and hormones are surging...maybe the Orp is thinking about going broody, thus acting differently and drawing attention.
I really don't think it's a breed issue. Even if you had a flock of all EEs you could still have an issue... or conversely you could have all different breeds and nothing happens at all. At this point I'd advise against adding any more bird(s) to the mix as you're already dealing with pecking order issues and frankly the run isn't big enough to be adding more birds (and doing a single integration is tougher).
As far as space, while you do meet the minimum recommendations on here, the run is very empty, so there's nothing for them to interact with and nowhere for a bullied bird to hide to get some relief. Additional space (which you mentioned was possible) and more importantly, clutter (which eats up floor space) might help bring some peace to the flock. https://www.backyardchickens.com/threads/a-cluttered-run.1323792/