A lovely, massive tank for sure.
I have certainly seen a massive tank at one of my local Maidenhead Aquatics which has a number of big, "aggressive" fish in, and with all that space, they are fine with each other.
It's hard to judge quite how much you might, or might not, be pushing your stocking levels without actually seeing pics of the tank, as it is so much bigger than the numbers we are normally used to dealing with.
Even so, I would be very careful about adding another flowerhorn. I would also have some concern about the flowerhorn you have if it is a male and starts to approach the size of the other big boys. Flowerhorns really are the next "league" up in aggression over any of the others you have.
If adding other large fish, I would try and avoid having breeding pairs, as this always has extra potential of causing aggro in your tank.
As for potential extra tankmates, you could consider any of the vieja genus or even some severums, which I think would fit in well, with that much space. If you were happy mixing from other continents then a paratilapia polleni or even frontosa might fit in OK too.
Personally, I would avoid any midas, red devils, festae or more jags as these species can take up a lot of gallons as their own personal territory. I'd say one of each of those would need about 100 gallons of space for their own territory, maybe more. I'd avoid a gold saum (green terror) too, as they are too similar looking to JD's and there is often big aggro between the two species.
How about a big veija argentina?
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