15:9 wont work, in veg MJ needs 18 hours minimum of light, so try 18/6 if not 24/0.
the heat lamp should be fine to keep in there, since one flo tube shouldnt produce that much heat...depends how hot it gets in your town i guess...
you should also add a fan in there if you're gonna use the heat lamp, or the leaves will curl and die from too much heat exposure