480g, 10ft long, with a 180g sump and 22 fishes total with 8 of them under 6in. I think your tank will be fine

I would be more concern with the aggression more than anything else. And yes, you can add a rtgg too if you want

My tank is half yours in gallon and I think I have about 18 fishes total and the smallest one is 6inches and some of the bigger ones are: 18in rtgg, 16in red devil, 16in gg, 12in rtgg *yes, I have 3 gouramis living together, the little one gets chased around once in a while by the big one* I like to buy them small and play with them in my smaller tanks, then when they get big I put them into my big tank. Everyone seems to be doing well. 2 FX5's and 1 eheim 2217 for water polishing/UV purposes, ammonia, chemicals, etc. are all ok. I feed every other day. With your 180g sump and adequate return flow, I think your filtration will be a lot better than mine.
Post pics when done