75g Tank Stocking List?

I would advise against using the sand and use only the pebbles. The sand, although layered at the bottom, will eventually be mixed around and then can cause you several headaches. One problem is that all cichlids like to dig around in the substrate to some degree and doing so will cause some of the sand grains to be swept into the water column and eventually into the intake tube of your power filters. Before long the sand can aculate and cause the impeller to jam and also scratch the magnet that drives the impeller. If the scratching is deep it can decrease the speed at which the impeller will turn and thus effectiveness of the filter in general. Your corys might also contribute to this as they mull around. Another problem is should you ever decide to vacuum the substrate to remove aculated debris, you will yourself do the same thing in addition to vacuuming a lot of the sand out with the debris. I have many times, while in the middle of vacuuming my tank bottom, had to stop because sand and gravel clogged up the siphoning hose and the clogs are sometimes difficult to remove. Finally, having sand on the very bottom layer can over time cause the sand to become packed very tightly so that water doesn't circulate through it and this can cause a sort of dead zone where harmful bacteria can live. Although not very likely, still a possibility you wouldn't want to wager the lives of your fish on.