Look up videos on topping and scrog. Start topping as soon as possible and get a net in there as soon as the stems are able to be moved. Switch to 12/12 in as little as 3 or 4 weeks. Remember, they will stretch as much as 3 times their height in flower, so a scrog and early switch will help keep them away from the light. If you scrog correctly with topping, that whole tent area should be filled with buds, with plenty of room between the tops and the light.
Downgrading to a 250 w will reduce the heat, but you're also reducing the light output. The clearance issue is mainly because of just the heat. I know you already bought the stuff, but you'd be better off getting a 400 watt t5, or an LED. The plants can get closer to the light without worrying too much about the heat stress, and you'll get the higher light output - in turn, more yield. 250 watt is going to get you shit for yield.