you will need to raise and lower the light. Dont fix it to the ceiling: instead mount hooks and find something like a chain that can be adjusted as needed. The lights should be 18" from the tops at all times. 250 watts is perfect for the space. I would put no more then 1 plant wide. Dont get greedy. I'd say 4 plants lengthwise will be fine. This will allow plenty of room for them to spread out, which they will and not interfere with one another. If you find that you need more plants, you could stagger them slightly and maybe get 1 or 2 more in. I would not stack them linearly like 2 wide and 4 depth like a formation or something. I dont think you have enough space for that. You will be tempted to cram in lots of plants, but really, 4 will be all you need.

It will get hot too hot for the plants unless you have ac registers in the closet as well. The solution is to cut a hole into the ceiling of the closet and install some sort of duct which is either connected to the ac or to a fan to suck out the heat. If you haven't bought the 250hps consider some of the other light options like the large CFL's they started making a few years ago. Home Depot caries a line of 100 watt cfl's (not 100watt equivalent but 100 watt input) for like 17 dollars. 3 of those would put out more light then the hps would and be far less hot. So much less hot that you might be able to just install fans into the closet and not have to cut holes into the ceiling for ventilation. I haven't seen the ballast but I cant imagine it's all that expensive if they are selling the bulb for 17 bucks.