My veg room is 6x7x7 and it has 2 1000w MH with air cooled hoods heat is not a problem unless it's the middle of summer and over 90 outside. For me I have an 8000-btu window AC unit so the extra heat in no big deal. If you have good air in and air out. Use air-cooled hoods with glass you should be fine. I run 12 to 20 at all times in the veg room so your number should be good to.

Peace