1. It is possible that merging with Samehada is not unique to Kisame. Consider that the blade's previous wielder, Fuguki Suikazan, also had a very fishy appearance. This may have something to do with the ability to merge with the sword.
2. Samehada absorbs chakra. In theory, it could absorb a tailed beast bomb or a rasengan. However, we do not know the rate at which it can absorb chakra, nor the capacity it has to store it. So, either of those techniques could be more than Samehada could handle.
The short answer to both questions is "nobody knows".