They can get a good size so a 75 would be better. 55 would be fine till at least 7-8 inches though (which is going to take at least a year or two).
They really don’t act too far off from any of the more intelligent cichlids - always going about the tank or motionless in one spot. Very personable, mine learned to recognize me for food within 24 hours of me owning it.
Being a hybrid of Midas and Vieja melanura, they have an attitude but this varies a lot between individuals. Mine is pretty mild mannered, and their messed up mouth limits a lot of their damage potential. Mix it with the right tankmates and it will do fine - I can’t comment on festivum as I haven’t owned one.