T8 and T5 bulbs have diferently spaced prongs on the endcap. The ballast may support another type, you would have to look it up unless it is labeled. I would just assume it is not. There is also a difference between T5 and T5HO. I have a 48" fixture of T5, and I bought an HO
ballast for it, for some reason it will overdrive the normal T5s but not light the HO
bulbs. Probably has something to do with the placement of the ballast. You also need the right relector, not just for light, but to fire the bulbs.
I would just buy a T5HO fixture.