Discus don't do well mixed with other fish because they like certain setups.
Most other cichlids are very aggressive, and again need specific setups, with other cichlids of that region or "lake." the semi-aggressive still get territorial.
Most sharks are aggressive. Balas need a few, and 3 is about all you can have in a 120, as far as I'm told.
Apollo sharks can be kept 3-5 in a 120, but you can't have any other mid or top swimmer at all.
You could have a redtail shark, but make sure it's tank mates are tough because it gets pretty aggressive, too.
It'd probably be better to chose one type of fish as the main type you like, and choose other fish from there that would get along. I know some nice tropical fish, but know nothing about cichlids other than the complexities of keeping them.
One common pleco can be housed in a 120, but I HIGHLY suggest not, as it limits your bottom swimming capacity. I'd suggest another large, but not 18", pleco. Maybe a Royal Pleco?