I think he should be OK, mine never really got mean to other fish really. Once they get to breeding size they will create a nest area where they hide. What they do then is they will gather macro algae to surround the opening and hide there nest area. Only males will do this from what I understand. That is when they will get a little territorial. Mine had done this a few times, cause after awhile the algae would get out of hand and I would clean it out. I have never seen a fish throw a temper tantrum like this fish did. He started breathing really, really hard and knocking stuff over and just went berserk. That lasted for about a day or 2 and he calmed down and started over again. That was the only time I did that after seeing his reaction.....My wife and I just sat there and laughed our butt off at this fish. I would say it was by far the most beautiful fish I have had.......
You 6-line should be fine, I had a mandarin in with mine and never had any issues...
