6 cubes/day would only be 72 square inches. Your 12" x 48" screen would technically be big enough for 48 cubes/day of feeding, if you had enough flow and light. Even with the old guidelines, there was a point of dimishinging returns w/r to growth, which is one of the things that triggered the guideline revision.
Starting with a 6 cube/day scrubber IMO would be the way to go. It will likely be a while before you start to exceed that capacity, and with larger tanks you really should attack filtration from multiple angles, so other components will take on their share of the filtration capacity over time.
But back to your original question, you do need T5HO as standard T5s don't really have the output density you need generally. For a 48x12 scrubber you would want almost 600W of total light, which is 300w per side, and with a 48" 54W T5HO lamp that's 6 per side, or one for every 2" of screen width. But that gets pretty ridiculous when you factor in reflector width, so realistically, you'll be able to get 4 in there at 3" width per reflector.
For a 72" sq in screen of the same width (12") you're looking at 6" tall x 12" wide and that's much easier to deal with. 2 18-23W CFLs on each side will do you very well, and you could actually use the floodlight CFLs also. Alternatively, you can use LEDs if you are handy and can do the build yourself, or you can use a stock fixture - but you have to be picky about which one you get, they are definitely not all alike.
