Do fish grieve...idk? But I have had fish re-act solomly after the loss of other fish. My Jewel Cichlid just hasn't been the same since that little hybrid fry (not related to the Jewel) died. My male Swordtail looked sad after my female died, as did my sister's male molly when her female died. Back when I only had 2 lemon tetras (these are a schooling fish, but I wasn't aware at the time), the tetra seemed to be sad after his buddy died.
Fish certainly react to other fish's deaths, but I don't know if they grieve or not.
About your Rosy barbs, these are also school fish. They must be kept in a group of at least 6. Without the proper numbers, it can/and sometimes will stress them to death, or worse, they get a contagious diesase that hurts/kills your other fish (like my lemon tetra back when I had 2). I recommend adding a group of 5-7 rosy barbs if your tank will allow it.