What about the green severum and a spectibilis (not sure if I'm spelling that correctly). I know they get rather large (10-12 in) and have a large bioload but they seem to be quite personable.
My opinion would be "go big". Get a single large personable cichlid because those cichlids tend to be the meanest with other tankmates. You aren't likely going to find anything as personable as you are hoping that will live peacefully with a sev. Sevs are gentle giants IMO and, in general, are easily bullied.
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