Its a fantastic tank and I dont think the fish you have had suggested come remotely close to a good stocking plan..
The Flowerhorn - depending on the fish they could claim the whole tank and not let you have any tank mates. Others could be ok but for me a Flowerhorn in this kind of tank is not my first choice. I would more likely suggest a large 4 foot tank for a Flowerhorn, so you end up with the incredible fish and able to plan a one tank fish rather than end up no choice but to do this.
The BGK will work in this tank and it is probably one of the first tanks I have seen in a while on this forum that will work fantastically for this fish
I like DerpPHs idea of the Geophagus Surinimensis - or rather the Geophagus Altifrons you find in the shops. Though there are quite a few species in this group that will work things like Geophagus Albios, Geophagus Dicrozoster, Geophagus Megasema, Geophgaus Winemilleri and Geophagus sp. Red Head Tapajo - though there are many more species.
You could then mix this as a large scale South American tank with things like Severums, Acaras, Chocolate Cichlids and maybe Pike Cichlids if you can find the right species.
The BGK will fit in great in this tank but you could mix it with other SA oddballs like Headstanders, Lima Shovel Nose Catfish, big Disk Tetras, Hemiodus. Big plecs, like Commons, Sailfins or one of the rarer L numbers like L14, L095, L152, L190, L191
Or if you dont mind mixing continents/ countries you could look at things like the Clown Loaches, big Synodontis, Bichirs.
What do you like the sound of?
Wills