Hey Jack,

Basement grow rooms are usually very nice, have usually a cooler temp and more consistent. In a basement, make sure you try to build it all air tight and use some sort of inlet filter; basements are notorius for small crawly bugs, inscects, gnats and even pathogen mold.

To determine the veg box height, figure out what size pots you are going to use, plus how much the light takes up and deduct this distance from the overall height. You talked about growing short plants, like 2' hi, so that with a 1' tall pot is three foot. T5 fixture is just a few inches, call it 6": you could use a 4' high box & a 5' tall is more then enough.

BTW consider mounting the T5 fixture or mounting the end caps in the top of the box. Move the plants up to meet the light, and when they get bigger down. In other words, adjust the plants not the lights.

A light rail is a motorized device to move the light back and forth along the long dimension. In you light diagram you show 2 rectangles repping the 2 light deal. 1 Light running between those two spots would probably work.