You can get any of the packaged sterile gravels and pebbles that the pet stores sell. Some people like natural,.some a certain color.
As far as stocking the tank with 3 good sized compatible fish, I would start with a Severum or Festivum, two peaceful and attractive cichllids. Also a Giant gourami, which can get a food long and is Strictly a surface fish, and a bottom feeder. The common pleco is rather drab, but the royal plecostomus with tiger stripes and Red Eyes is a good bottom fish and tank cleaner. If not a Royal pleco, try googling "Images for Synodontis" catfish and click on the caption.and you will see a vast number of species. the Upside Down catfish, S. nigroventris is very popular, as yes it does swim upside down a lot.
As an alternative, you can get 3 or 4 African cichlids. these live in alkaline water with pH over 8. you use crushed coral gravel for these, and since no plants will grow at that pH, you decorate with coral heads or conch shells. Among the easiest and most personable are Frontosas from Lake Tanganyika. they need a lot of corals and a rockpile, as each one digs its own tunnel. they get over 6 inches long and live in family groups. they will adopt smaller ones. they are mouthbrooders who have only about 25 fry and raise them in their mouths until they can fend for themselves. You can keep them with Peacocks, Lake Malawi Cichlids who eat a lot of vegetable pellets. Frontosa like pieces of shrimp and Medium sized pellets, especially omega's with garlic. You can keep a Synodontis decorus catfish with them. You need to keep the pH up, which you do with baking soda, about 1 ng teaspoon per 10 gallons starting with neutral ph 7 tapwater. or cichlid buffer which has a few other trace minerals for good measure. You need to test the pH about every 2 weeks in a big tank.
Robert Price, Phd, Ichthyology