and the first section is going to clog up.. having the fine media there for polishing is going to be wasted by then sending the water through the rest of the media... and the heater section is a huge waste of space..
Put mech. filtration from coarsest to the finest polishing pad right before the bio-medium (keeps crap out of the bio-media). Go ahead and mix your bio-media as different types of bacteria prefure to inhabit different surface's, so the bacteria that are generally found on scrubbies aren't necessarily the same kind that are predominantly on a sintered glass media.
Instead of the heater area, just use that as the pump area and just place the heaters in there with them. If you want a deep sand bed for presumably nitrate removal, i doubt its effectiveness will really be all that noticable, but just get a deep narow plastic piece of tupperware or something of the type and fill it full of sand up to 1" from the top and place it in your heater/pump part of the sump.
work out some kinks and it doesn't look like a bad setup at all.