They need a long tank with wide room to turn, though 12 feet would be good, I heard something of a 9 foot long, 4 foot wide tank would be a decent minimum.
Of course, I doubt any of them really get such a tank.
I have a 6 foot 120 with 5 balas, 4 silver dollars, 2 African fins and a bunch of corys. They all get along and are doing great. I do plan on upgrading next year to a bigger tank.
i have 2 balas now in a 55 gallon just got a 125 that needs to be set up but the 55 right now is fine for them there about 4 inches long and growing fast on brine shrimp