Yes, the fixtures are often sold with the 5.0 t5 or with no bulb at all. The 5.0 will still work well if you mount it under the screen with picture hanging wire or something similar. Here's how I mounted a t8 fixture under screen.
Ys, t will be fine on top, no uvb blocked if you have a screen with 1/2 " holes. He would get even more uvb with it mounted inside if you feel the need but the t5 5.0 is still a good bulb for uvb. It's the equivalent of the t8 10.0
So, I was gifted a fishtank a long time ago with a couple lights in it, and I have had them stored in my basement for a while now. I just looked back at them, and turns out that one of them is a T8 18" fixture, with a T8 bulb in it!! should I keep the plastic guard in front of the bulb, or keep it removed?