I wouldnt go with the duel spectrum unless you have a moveing track for the
lights so you can evenly distribute bouth light spectrums evenly throughout
your crops(your not getting one in there)I use just a lakewood fan blowing across the tops of my plants inbetween the plants and the bulb.
I would suggest a dehumidifier... it builds up like a bastard in there
As for how many plants you can fit in there...... it all depends what your final goal is.. what kind of yeild your looking for also what time you start flowering your plants.... in the 4 x 4 x 7 model of the hydro hut you cant fit no more than 4 LARGE plants without it being rediculas. And to finish again if i were you dependng apon how many plants you plan to grow i would go with 2 or 3 250w hps with mh conversion bulbs for veg then get the hortilux enhanced spectrum bulbs for flowering and they will be very happy. I say get 2 or 3 small ones because i had found with my 1 600w hps i am haveing a problem covering a 4 x 4 area good. I think i might be picking up another 250 soon and grabbing those mh conversion bulbs myself.

Hope I could help,
Cptweedmanguy