Aquarium supply places sell little clips for raw vegetables, and they'll probably sink once they are waterlogged. You can also buy sinking algae wafers for the pleco; since he's a night feeder, you put in the wafer after lights out.
Some fish continue to grow throughout their lives, but most are considered mature at about one year. Aquarium shops will sometimes trade your large fish for more suitable species. The pleco and oscar will get too big for almost any aquarium, so you may want to trade them in right now. The terror guys are not the best of community fish, so you may need to trade them in too if you want other species. An excellent community fish is a cory cat; six will put on a good show. If you want egg layers, consider zebra danios or white clouds. Cardinal and neon tetras in shoals of at least six are very attractive, but they're not the easiest to keep healthy. Gouramis are also popular, but some get pretty big and they are sensitive to diseases. My favorites are angelfish, but they can become quite a handful and are compatible with almost noting but cory cats.
For live bearing fish it's hard to beat guppies and red wags. If your aquarium has plenty of floating plants, a pair of wags or guppies will fill the tank in a few months.
No doubt that's more information than you asked for.