My 55 african biotope has 2m/6f congo tetras, 3 buffalo head cichlids, 1 upside down cat and a single ctenopoma. It's a really nice tank. I'm thinking of getting 2 more ctenopoma atleast until they get some size, it's about 1.5" and a few more synos.
I would do atleast 3 ctenopoma, that will keep any aggression spread around instead of concentrated on one fish.