T rating is eights of an inch, a T5 bulb is 5 eights of an inch around, a T8 Bulb is 1 inch around. What bulb you use in what fixture only matters if the specs match up. Example, if you have a T5 Ho Fixture that is 4ft long and uses 54watt bulbs, you can use ANY 4ft T5 Ho 54w Bulb in it, be it standard house hold lighting, be it Antintic saltwater reef tank bulbs (Antintic Bulbs are a great source of nothing but blue light spectrum by the way), or grow lights.
Cheers that explains what I needed to know, I was surprised I couldn't find it anywhere in the lighting forums on like 4 different sites.
Not a problem man, i am full of utterly useless knowlege, its nice when some of it actually becomes useful!