An 18,000K Aqua-Glo is a T10 bulb more than likely. It can usually be used in a fixture designed for T8 or T12 unless either have bulb-fitting waterproof endcaps. T5's take a different pin size plus are actually shorter by an inch or two and wont be interchangeable, so if you get the T5 unit you will need to buy T5 bulbs.
If you went any more than 2x54W T5HO you would be needing co2 and a good fert routine. But what many do is get something like a 4x bulb fixture and only run two, in case later on they want to add the co2 and stuff then they can flip the switch and run 4 bulbs. Or with a 4 bulb unit you could do a noon burst for a couple hours at full intesnity. Again, I'd only do that with co2 and ferts though. I'm not familiar with the Nova units but if they have good reflectors in them then even 2x54W and your bordeline on needing supplimental co2 and ferts... do you plan on having your tank more full of plants?