Most plecos will fight for territory against other bottom dwellers, especially other plecos. If you give royals plenty of driftwood and hiding places, they shouldn't bother other fish, including your bgk.
I personally don't see him bothering the Knife. I kept several plecos with my knife and never had an issue. If he hasn't shown any real aggression towards other fish aside from plecos, I couldn't see why he would mess with the knife.