Poppy and Bruno get into little brawls if Pebbles comes near the front oftheir cage. For the most part, neither Poppy or Bruno likes Pebbles, so they get quite upset when they can see her near the front of theircage.
Are they in the same room? If they are, maybe try covering the part of the cage where they can see the fosters with a sheet possibly? For me, as soon as I get Pebbles away from the front of their cage while she's out, they go back to grooming.
It might just take some time for them to get used to. After Bruno and Poppy bonded, I moved Pebbles down right beside them with double grid so they couldn't bite through the bars, then put a sheet on their 'fence'. Once they got accustomed to Pebbles smell, they settled down enough for me to take the sheet away. They are now living happily together right beside each other with no sheet, but the grids overlapped so theirs no biting.
Hope Fred settles down!
