If you want to ad another fish, I think you should wait until you get the 150, and move the bifas and the others in at the same time. Your bifas owns the 90 and the entire tank its in, is the bifas's territory, meaning any other cichlid will be a threat, and the bifas would naturally want to kill it. If you wait until they can be put in a new tank together it may go much smoother. I think a festae would work, if you do it the above way.
