Whoa whoa WHOA.
This bala guy was dead on when he said that balas will only grow to the size of their tank. Their skeletons will, their organs will not. In essence, they get crushed to death from the inside out by a condition caused by stunting called organ compaction. Why do you think he keeps his balas in a POND?!
Also, what exactly defines the size of the tank. Just because their growth will be stunted does not mean they can't still grow larger than what your tank will support. In fact, skittish balas (from being kept in inadequate schools) have been known to bolt into tank walls and the lid when they get startled, potentially breaking the tank.
Furthermore the bioload of even 2 balas and a pleco alone will be too heavy for your tank, stunting or no, poisoning your fish with ammonia, nitrite and nitrate (ie their own waste).
As for the tetras, 6 is the minimum they can properly be kept in, but a school of more is way more likely to work out successfully in your tank.