I would advise you not to use the 600w HPS as the generate a lot a heat and light too, you may just stress them... I recommend changing you're incandescent light for a fluorescent one as they generate less heat and use less energy.

What is the temperature of the cabinet right now ? You should always be prepare to get quite a few degree up with the HPS running, this is the reason why you need to have a good exhaust.

Do you have any idea how powerful you're fan is, you should be able to exhaust the entire cabinet in less than 5 minutes to be sure to supply enough Co2 to the plants and get the heat from the HPS out ASAP !