I'd take a tender/oars if you can. On Bryher I think Bennetts boatyard will fetch you from the moorings, but most of the other islands you'll need to get ashore yourself. No pontoons to leave the RIB on long term, although all the islands have town quays you could use just to drop people off.
We love drying the yacht out on the sandy beach on Green Bay (Bryher), which is one of the most protected anchorages in Scilly, and then you could walk/paddle ashore! There's also some good moorings in Porth Cressa (the south side of Hugh Town) that are sometimes a bit more protected than Hugh Town itself, and a lovely anchorage at Watermill Cove on the eastern end of St Marys. I love the various bays on the north side of St Martins as well.
Very jealous, we had a great trip to Scilly last year, admittedly by sailing yacht, but I'd love to take the RIB there one day.
Have a great trip!