Bala's do best in groups of at least 5-6 and need at least a 5-6ft long by 18inch wide and 2ft tall tank minimum, these sharks grow big with the potential to grow to 12inch+ and are very active and skittish fish, so the larger the tank the better. If you do not have a tank of this size right now and do not see yourself getting one anytime soon, you should really rehome the sharks now before you get emotionally attached to them. But, if you have properly researched these fish and planned ahead and can give them a suitable habitat/home to live in, then i say well done

!
The Red tailed black shark on the other hand in many respects is a very different fish with different needs. These fish are not sociable like bala's, they are solitary fish and are very territorial and agressive towards their own kind. They are not the brightest of fish are times and will often see other fish which resemble RTBS in shape/appearance or colours as a threat, thus it is strongly advised that you do not mix RTBS with other shark-shaped fish or fish which have similar colours or patterning to RTBS as more than often RTBS become very agressive and territorial to such fish and will end up harrassing them non-stop, which usually ends up with either the RTBS stressing itself out too much and dying, or the persecuted fish getting too stressed and dying.
Stressed-out fish are not good as fish become a lot more susceptable to illness/diseases/parasites etc when stressed, which can put the whole health of the fish community in the tank in danger if fish are carrying diseases and parasites and stuff.
If you have a large enough tank or are definately getting a large enough tank for a shoal of bala's in the near future, i would stick with the bala sharks and rehome the RTBS, on the other hand though if your tank is not large enough for the bala sharks but only large enough for a lone RTBS, i would stick with the RTBS then and rehome the bala's. RTBS need a tank of at least 3ft in length and at least 29-30 gallons or more.
Bala sharks do well with fish which will not freak them out, as bala sharks are already fish which are naturally very skittish in aquariums. So tank mates which keep to themselves and/or are peaceful and enjoy the same sort of habitat etc as bala's make good tank mates for bala sharks. There are loads of fish that all into this category, so you kinda need to give more info as to what sort of fish interest you etc.
RTBS do best with tank mates which are not likely to upset the RTBS- you should take into consideration stuff like how RTBS will often target fish which look similar to them. RTBS vary a great deal personality-wise...Providing the RTBS with a suitable habitat (i.e. lots of caves and decor to break up view point in tank etc), diet (mostly lots of algae/veg) and a good sized tank is crucial in making sure your RTBS does not become very agressive/territorial towards other fish, as the more agressive/territorial they become, the more difficult it is to find suitable tank mates for RTBS.
RTBS are generally not very agressive/territorial when juveniles, but will become more so as they mature. But this varies a great deal since RTBS themselves vary a great deal in personality/character themselves etc.