OP: The only issue I have ever noticed with keeping multiple BTA's is their propensity to collect in one area due to conditions and one may out compete the area, however if the tank is setup specifically for them there should be several suitable spots and they should be fine. Keep in mind that at full size these guys can reach 18" so 7 in a standard 55g is not as large as you might think (my opinion is that it is plenty large for what you are doing).
Bradguy: "T5 lighting" is not terribly descriptive, how many bulbs and what size tank will greatly factor in as well as what combination of bulbs. "Good flow" is also rather generic, GPH from power heads and where they are directed is important, and in my experience BTA do not seem fond of violent/turbulent flow but instead like a sweeping current that they can extend their tentacles in to. As for parameters, BTA in my experience are more tolerant of high nitrates, and sometimes phosphates, than they are of drastic swings in nitrates, phosphates, calcium, alk, and ph. Keep in mind that "fleshy" swelling and deflating that nems do is dependant on them pushing water and nutrients in and water and waste out, so they need consistency to be able to do this efficiently.
