I don't see any problems with using any of your tubes, bellows or lenses on the D5100 with the exception of the BR2 directly on the body. It's OK on your tubes and bellows but may interfere with the lens contacts on the body. There is no TTL with the SB21B on a digital body. You'll need to use it in manual mode. Whatever accessory or lens you attach to the body only the AFS and AF lenses with give you metering. Based on your profile it looks like you shoot a lot of close up and micro photography. Except for metering you should be fine. The articulating LCD and Live View are great for close-ups.