Personally, i would keep it to 4 GBRs as they are very territorial and can get aggressive with each other. I would probably advise trying to get 2 males and 2 females. I would get a solid bunch of about 4-5 guppies, 6 Panda corys, and as many shrimp as you want. I would do either the guppies or the tetras, but doing both seems like a little much.
I personally have a GBR and i have had no problems keeping it except that he is picky and doesnt like to eat flake food, but he sure loves brine shrimp. He is very peaceful with my mollies. A great fish and i cant wait to get a female to join him
20 Gallon High- Diy co2, 48w, flourite. Still new to this whole planted tank thing