You're right, they're often listed on various links online to reach some 5-6". However if you did deeper to anyone who's had these fish for years and/ or is breeding them I never found nor heard of anyone who's fish reached this size nor found reliable articles anyone founded these sizes naturally out in the river.
The greatest I ever heard of was 3" and its safe to say your every Rosy in the tank under good care (as always) will reach around 2-2.5".
The key to most stocking is in the set up really, this doesn't mean you can stock unlimited, but you you really go well planted tank, some caves for the cory etc, so everybody can get outta 1 another's way every once in a while you'll do just fine. Kinda like most people, we love being around people 100's of them, but sometimes its just good to go home, shut there door's and leave the world outside, right!?
If you're looking for plant ideas just check my tanks out, see what you like, if I rattle my brains enough I should be able to name almost every plant there
Well and then apart from the set up f cause that they'll get along (non aggressive) and proper maintenance...