There ARE both NO T5 and HO T5! Take 48" bulbs, for example, HO T5 is 54w, NO T5 is 28w, both bulbs are available. So if you can find a HO T5 the same length(I don't see anyting other than 8w NO for 12" though), then your ballast(assuming electronic) should be able to drive it.
HO and NO T5...