I have kept 2 species from the gymnogenys group, sp. Rio Yerbalito, and sp.Paso Pache.
Never experienced aggression problems with either.
I never used tanks for a group, any smaller than 6 ft , and only kept one species of the genus per tank..
The groups were always 1 male, to at least 3 females, and if a female was holding, she'd get her own tank.
I never used heaters in their tanks, just going with room temp.
Above are species "Paso Pache'
Inside they were held in a 125, using Mexican high altitude live bearers as dithers fish, they were also kept outside in a 300 gal rubbermaid where they spawned.
Below species Rio Yerbaito, kept in a 125, but female moved to a 50gal when holding
They were also kept in a pond spring thru fall, brought into a 400 gal kiddy pool in winter
I did keep a group of Rio Yerbaito in a 125 Uruguayan cichlid community tank, with Australoheros "red Ceibal, and the Uruguayan pike cichlid variant of
, Crenicichla saxatillis. It was on a north wall, with no heater.
When brought in after a summer outside in the fall colors were quite striking.
Male left, female right.