Hmmm.....1k for vegging fifteen plants? Soil or hydro? It makes a difference. Hydro you have less cooling options. Loc gave you some good suggestions. You can also reduce heat by turning down the light. I run a 1k ballast on 500 setting for my vegging. I mean it's not going to kill the plants to give them less light. If you don't have it ducted to the outside you're really going to struggle with the heat. Spend your bucks on a good fan and go to your home store and get the ducting. I used styrofoam insulation board to seal my windows and cut holes to mount the intake and exhaust ports.

How you set the vent up depends upon your goal. Heat rises and you want your exhaust above your intake to give you maximum benefit. If your shed is like mine it probably leaks a little air. If you're going to run CO2 that requires more cooling as the room will need to be more sealed to enjoy the benefits of the gas.