If you have 2 of any of the cichlids, you could end up with a breeding pair and they may end up beating up on your other fish. Unless you are looking to breed, I would avoid getting pairs.
I typically like building my tank around a showpiece fish, then stocking dithers, top dwellers and bottom dwellers.
Here's how I would stock a 300G:
(1) One of the less aggressive cichlids as the showpiece
(5) Apollo Sharks- Luciosoma spilopleura (surface dwellers)
(8) Filament Barbs- Dawkinsia filamentosa (Dithers that swim everywhere)
(8) Clown Loaches- Chromobotia macracanthus (Bottom dweller)
If you are buying all of these fish young, get the cichlid last so that the others have time to put on some size and become comfortable in the tank. It's always best to add the most aggressive specimen last.
Everyone has their own opinions, but I'm not a fan of cichlid only tanks. I don't really enjoy watching fish beat up on one another and posture back and forth trying to claim the limited space in the tank. I really don't like it when the bullying gets to the point of fishes fins be shredded. By going with one cichlid, he will be the boss of the tank. The others will add lots of color and nice relaxing movement....much more pleasing to watch than the bravado of a cichlid only tank.