Ditto on Rosey barbs being hardy... I have 3 in my 29 gallon and they are VERY active chasing each other around. I also have 17 Neon tetras and there's never been a show of agression towards them. Yes, definitely a school is needed as I bet if one is left alone, it'd prolly chase the less agressive tankmates around.
My water is typically 6.8 PH
, 76 degrees F. Mine haven't grown past 2 inches and I've had them for about 2 yrs now.
They nip at my weaker plants.... that's my only complaint about them.