yes white saum costs more and is harder to find. other than that they are both the same fish. just different localities. I remember reading on here that a member of the forum went collecting in south america and found the gold and silver saums together in the same area. so I suppose they are just variation, like the difference between a red red devil or a whitish orange one. same fish different variety.
also the first described Green terror wasn't even what we keep today, it had white edging but was a different species to both the gold and silver saum.
as for in a 75 the width is what gets you, if you have a wide tank its better than a long skinny tank. so if your tank is only 18" wide keeping a 12" fish in it would be kind of cruel unless it was the only guy in there.
I used to have a 3x2x2 breeding tank to breed large cichlids in and never had any issues with that.
Salvini
Nicaraguensis
Texas
geophagus species
firemouths and other thoricthys species
honduran red point (fish in my avatar)
green terror
smaller parachromis species, I believe either friedrichstahli or loeseli (can't spell) get around 8"-10"