Search for the growing guides on here. Closets are hard to grow in because the make ventilation difficult.
yes you can you just gotta have alot of fresh cold air brought into the area more then is being taken out and not have your fans directly blowing on your plants 24/7 oscilating ones are good.
I have 400 watt hps in a 4 x 2 space with no issues, it never gets over 70. Just don't skimp on a fan, get one bigger than you will think you will need.