Most people that have seen "aggressive" behaviour are keeping too small a group in too cramped quarters. They are not typically aggressive. They grow large and are very active, so their bulk at speed will frighten smaller fish, but then, it is rarely a good idea to mix fish of greatly differing sizes.
I have seen them up to about 125mm, most "adults" appear smaller, perhaps they are not adult. In excellent conditions I would expect them to grow larger. FishBase quotes 150mm which I would not be suprised to see bettered in the wild.
The price has come down. You don't say where you are, which doesn't help. Monty at Tri-Mar in the UK sells them for ~£10 the price fluctuates a little with availability and size.