Just a single small CFL is enough light for a cloner. Too much light, and they grow SHOOTS, not ROOTS.

I think if you want to take your plants to 18" in veg, you are looking at a whole different ballgame. Even a good T5 flouro will have trouble penetrating that canopy more than a foot or so. I used to bring plants to 10-12" in veg under shop light T12s and that was definitely as far as I'd recommend going with tubes. Maybe 14" under T5s? This is speculation though; after plants hit a foot I either flower them, throw them under my MH400, or toss em in the rubbish if I've got no space, lol!

Remember there's lots of ways to skin a cat, and you can configure that closet any way that works. Hint: when you hang the mounting hooks in the ceiling studs, hang EXTRA ones, and attach lengths of chain hanging down so you can easily add more hanging stuff (lights, fans, ductwork, power cords) without having to remove EVERYTHING from the closet and go hunting for your stud finder and power drill!