I notice that you do not know about cycling. I think that you first need to read about that, since you have over 6 feet of fish. Do you know how many gallons or liters this is, since balas need at least 55 gallons, or how tall it is?
Please read this
http://www.fishlore.com/NitrogenCycle.htm to help you with cycling... If you do not properly cycle this tank, you will very soon notice lots of dead fish...
And the so answer to your question before is that if you have a big enough tank, which it doesn't sound like, and you are finished cycling, which it doesn't sound like, then you could put in some neons. Just remember that there should be 1 inch of fish per gallon, plus extra for the biggies like balas...