Just a few comments. My sister has a red-tailed black shark that lives in a 3 foot tank with some gouramis of all things and a flying fox - she has had no trouble at all in terms of aggression - even with the other cyprinid.
On the other hand we have a small rainbow shark (some say they are less aggressive than rtbs) and its harassing everything we have in the large four foot 200 gallon tank.
Ist just their nature, but i doubt either a RTBS or a rainbow would be suitable for ram cichlids - aggressive african cichlids would be better.
Last comment - I often hear that Bala sharks need to be in a school to be happy. We had one that is about 8 or 9 inchs that attacks any other bala shark it encounters. We were taking care of it and returned it to its owner and in its new tank it started attacking the other balas - and this guy owns a HUGE tank. They have a rainbow that attacks all the balas, and a bala that attacks all the other balas, a chinese sucker that attacks all the other cyprinids and some tinfoil barbs and clown loaches that try to avoid the charks.