If you dont want to use gravel, play sand will work fine (its what i used for my DSB in my sw). you will probably need three or four bags.
I would just take the cartridge out of the filter and rinse it out in the tank for some help, but you will still probably need to cycle.
and, if you're going to put all that rock in there i wouldnt use the driftwood (it would look really funny). african cichlids would be the best choice for that decor.
75 G. SW FOWLR - 3 Green Chromis, Talbot Damsel, Sailfin tang, Emperor angelfish, peppermint shrimp
29 G. community FW- 5 redeye tetras, 1 bloodfin tetra, 1 red platy, 1 angel and 1 black veil angel, 2 cherry barbs.
10 G. planted tank, lots of Cherry shrimp