6 balas in a 120? Balas get huge over time 12-14+ inch 120 is way to small for them imho.
6 rainbow sharks is not a good idea as they tend to fight amongst one another.
ID shark will out grow a 120.
2 common plecos will also be a very tight fit in a 120
the colombian sharks people have gone over.
The only sharks you could really go with are a single red tail or a black shark but even a black shark in a 120 is pushing it, if you do get a black shark I highly recommend keeping it alone once it gets some size.
You could get away with 2 balas and 1 RTS but the balas will be very skittish as they prefer to have a schooling group.