vegging with an HPS is not very energy efficient.
You can veg under CFL's or Flouro's quite nicely and by far it's cheaper on electricity.

You should be able to fit 2-6 plants in that space depending on strain of course and how long you want to veg etc.

Cab looks good though, temps seem manageable.

No need for 17/7 etc - 18/6 or 24/0 will do you fine. The switch to 12/12 should be when plants are 1/2 the height you want them to finish at.

Have fun..