I obviously don't have all the answers LOL
but I do have some suggestions ideas:
Non-fish. Theres a bunch of folk here who have crayfish and crabs; I'll defer to them on those crustaceans, but do let me suggest FW
shrimp. Awesome buggers. Ghost shrimp, Cherry shrimp, Amano shrimp all are bottom feeders of sorts, and are wee lil guys .5 - 1.5 inches average, although I know a few folks here (Madame_X I believe) have mutant ghost shrimp which are about 3 inches long. Bamboo shrimp are filter feeders, and would need a mature tank as they feed on microrganisms from the water column, but are VERY cool to watch and get a bit bigger (3 - 3.5 inches). The there are the atyas which are even bigger. All those mentioned are safe for other fish in the tank; they generally are too small or have filters instead of claws. You can read about em here: http://www.thekrib.com/Fish/Shrimp/
. I have neon/cardinals, gouramis, otos and cherry shrimp in my planted tank; I had a bamboo but he went carpet surfing; I'll be purchasing 2 more this week.
Substrate. You certainly can use a combo of sand and rock. The only thing to keep in mind, is sand needs to be aerated somehow to prevent buildup of anaerobic bacteria which gives off hydrogen sulfide and is deadly to fish. Plant roots, snails which burrow, weekly shifting/digging into the sand all help bring O2
into the sand and prevent that from happening. I have sand in my planted tank and clean it once a week. I use a smaller diameter siphon, and run it above the sand. The sand is heavier and stays as long as I'm paying attention LOL
and the detritus gets sucked up. Easy peasy.
Lastly for now, you sure can have live plants, depending on your fish. I have discovered I really can't do a true planted tank with my pleco; he digs them up and eats em. All the plants in that 55g tank are in flowerpots to prevent him from dragging them around the tank. He also chews on them, and my amazons in that tank look pathetic.
Good choice on the filter; you may or may not need the powerhead depending on the fish you chose. Some lurve current, some don't.