1.) give them time to grow another inch to an inch-and-a-half
2.) GT sexual maturity is typically 3" - the exact same time aggression developed through sexual frustration occurs - in some cases, all-female groups not finding a suitable mate so a dominant females takes root, resulting in the weakest female getting bullied for no good reason. You can try isolating the weakest or the strongest but until you get a male there's a good chance it will continue until you either get a male or have 4-5 tanks of individually housed female Rivs. (which SHOULD be ok w/ other cichlids, but just not other female GTs)
The female GT is not a fish to be screwed with. They can PMS like an Atom Bomb.....my female GT, "Angel of Death" chases my male Dovii constantly and she has no fear of anything, including Violator, my 9" male Jaguar cichlid.....and she's barely 4-4.5" if that
Meanwhile Nelo Angelo, my 6"male is the most docile fish in my collection. I have Rainbow cichlids more aggressive than he is. Both my GTs are housed separately until they reach max. size for breeding purposes