I've just added my 2 previously QT
'd rosy barbs to my 28 gal
tank. There has definitely been some fighting to establish pecking order. My original lone barb is much bigger than the two new ones, and not surprisingly is kicking some new rosy barb butt! The new guys are about the same size as an adult platy. One of them fights back, one of them seems a little wimpy and just takes the beating.
Anyway, three really fast swimmers in the tank has certainly changed the dynamic. I have one platy who is now hiding, and another sometimes-hider who is hiding more now. I haven't seen the barbs pick on anybody else... maybe the others just have to get used to their new buddies.