well, first off, your corals can live with some nitrates if you keep softies and lps, but i see you want some of the harder stuff, which i cant help you on, as i have never tried to keep them.
also, on the filtration, from my knowledge, you have bioballs in the back chamber. i would replace those with some live rock rubble. maybe add a fuge with some cheato too.
with that said, i think you could leave the sponge in there, but i would clean it once a week to make sure the nitrates stay low.
yea, seems a lot, and thats why i never have and probably never will venture off to the sps side of reefing...
good luck.