I would absolutely split my closet from side to side. Again, a long, thin closet is not ideal for lighting. The farther light has to travel, the less effective it is. If your grow are is more than twice as wide as it is deep, you're losing efficiency. If you've got a 5' wide by 18" deep area, divide it side to side. No question. If you divide it so you've got 36" x 18" deep on one side, and 24" x 18" on the other, you'll be able to flower three plants while you veg a bunch of others. It's pretty much a constant system. Just my .02 on that.

On a HID (High Intensity Discharge) light, the ballast is the machinery that keeps the power flow constant and level. This is very important with HID lights. Any variance in the current flowing to the bulb will cause it to shatter and may short out the light assembly. Since ballasts regular power, they have current flowering through them. That means they heat up. If your ballast is with your light bulb assembly, you've got more heat in your plant area. If you have a separate ballast, you can keep it outside your grow room, which means less heat.

Yeah, you could use the CFLs with the 150s and get enough light for 3 plants. And it's just my opinion, but I'd use CFLs in veg at first, too...they're cheap and they're better for the first couple of weeks. MH is pretty hard on plants at first.