I think Im trying too hard to have two set ups in one tank, Ill knock this idea on the head now and enjoy what I have now Once everyone starts growing its going to look great I just need to be patient........or buy a second tank
I've done black belts (Vieja Maculicauda) with some geophagus altifrons. The black belt just ignored the geos and occasionally chased them but nothing worse than a nipped fin or two. Maculicaudas are less agressive than argenteas from what I've heard (I don't own an argentea, yet)
I think you would be fine putting a Regani or Heterospila in there. Maybe even a synspilum. I know this isn't everyones experience, but I have a pair that is very mild.