You can do that if you edit its disassembly, but I couldn't find it in the sprites section... There's another Big Boo Boss sprite there, but I don't know how it works. Its descriptions says that it's based on the original sprite.
Not really. You're almost never going to need anything more advanced than basic addition, subtraction, multiplication and division (in fact, the SNES can't do anything more advanced than that without enhancement chips); a basic sense of logic is much more important that being good at maths when it comes to most kinds of programming. And just about every calculation you need to do is performed by the code you write rather than by yourself.
We have several ASM tutorials in the tutorials section and the tutorials forum; off the top of my head, I can recommend Ersanio's one and imamelia's one, although the latter isn't completely finished yet.