Balas are definately to big to house correctly in a 75.
A shoaling fish that can grow to 15 inches,it's a no brainer.
Most silver dollar species are also unsuitable in my opinion. To be truly happy they need to be in large shoals in tanks over 6 foot. In small tanks in groups of less than 6, yours chances of skittish fish are high.
As already mentioned, lippincotianus is probably the best choice but I would get no less than 8.
Red bellies grow to around 12 inches live in shoals and are messy feeders.
You won't be able to keep many adults in that tank.
In my experience small numbers of natteri produces a boring display of skittish fish that hide a lot and hardly move.
Personally I would have a re think.
As far as Bala sharks looking aggressive,do you really think so?
Nothing but the made up shark name is remotely aggressive to me.
Have you looked at dwarf snakeheads?
Some won't grow much more than 6 inches,look aggressive and are true predators.