Being a relative newbie I was pretty freaked out the first time I tried pairing adults. I had eight dk. marble super veils in a 150g. tank and they fought like crazy. Lip locking and pecking at each others sides...I was sure that they'd all kill each other, so I took the four roughest characters out and the others settled down within about an hour. The other four weren't pairs, just tryin' to decide who was going to be boss I suppose.
The tank was needed for others so I moved the calmer four and two of the original trouble makers into a bare 5og. and they all do fine. A couple of days later I reintroduced the last two of the fighters and there was about another hour of aggression before everyone settled down.
I have no idea what all of that meant 'cuz I'm not experienced with their behavior yet, but be prepared, their name belies just how rough they can get. I had a small black female take a chunk out of her mates head and kill him a while back, I think because he ate the eggs of their spawn. Scary little wench, he was twice her size.