I'm starting to think about taking the first area and splitting it in half to give 6"x8" to the bio balls and 6x8" to the skimmer. The way i have carbon now is in a bag under my bio balls, that's where i have a bag with some phosphate remover as well. I don't know if i'll still have to do that or not though.
my drawing (on paper) has 6" for the pump return, 1" between baffles, 12" sand bed, 1" between baffles, then 8" to play with on the intake. I have a remora skimmer, which is about 5 or 6" wide, but has a side discharge. I'm going to have to play with this a little. I can probably put the pump in about 4" space and give the intake a little more room, but i'm not sure what that will do to the pump.