Yep, with Rainbow's or RedTails, it's either ... also from what I've read ... keep one, or keep a group like 5+ for the same reason's mentioned. Keeping just 2 to a few could literally mean an all out brawl as they mature and their territorial aggression builds.
Personally speaking, I wouldn't want a group of mature Rainbows together in typical sized tanks. Get me a 1,000+ gallon tank and then I'd consider it.
In a 120gal, with enough territories set up by using DW, Plants and Rocks, you could get two to co-exist. Of course, there's no guarantees.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.