There won't be any proof until 1) the genus gets described (remember all members of the blue acara group, which includes green terrors and saums, are orphaned atm) and 2) the fish in question gets scientificly described. Even if they were orginally 3 differant species, they certainly are all now muggled together much like the differant convict species are. Right now it's all opinion until a scientist gets working on this group of fish.
Personally, I don't believe the slight differances in scale patterns and fin color between red saums and gold saums is enough to merit them being differant species, though obviously enough to be called differant color variations. But then I also didn't think convicts needed to be split into 4 differant species either ... so what do I know?
The few (2 only) red saums I've seen were deffinately differant enough in person that I would own one over a normal gold saum, paying more for the differant scalation pattern alone. I just think until the scientists sort it out, treat each of the 3 saums as seperate species (even though I don't think they are). If we are wrong, we are preventing hybrids that way and avoiding another convict mess.