I bought a 12" bgk and 16-18" clown knife and quarantined them in a 240g tank. It didn't go well. The clown knife beat up the bgk pretty bad. The bgk healed up and I still have him...about 14-15" now. I don't think it's worth it for you to add a clown knife.
As an aside, has the breeding pair ever had babies?
That is awesome! I had a large pair (never bred) in a 175g community tank years ago. This was when blue led moonlights just came out. I could watch them "dance" togethor for under the moonlights...so graceful and relaxing. Best of luck with the breeding project, I hope they pull it off!