Its the upper end of whats recommended for the Geos but I think you should be ok. Red Heads are commercialy bred now so most will be tank raised which while not solving the problem will at least help.Thats a very tough one to pick between as I really like the JD and Geo's aha. Would a blue acara fit into the SA tank? My water is 7.2 ph and 10.38 dh, would this be too hard for SA's?
Blue Acara would work, would definitely need to let them get to a decent size before adding the Oscar though. How fixed on the Oscar are you? They often just eat tank mates (sometimes of surprising size) and without one you would have some more options in terms of a bigger community around the cichlids. There isnt really an alternative to them as they are so unique in their shape, size and personality though, it would just be a different choice to replace them.
Wills