An alchemist can learn as many types of alchemy as they want in the same way a scientist can know both chemistry, physics, biology, etcetera. Of course, top level alchemists have their specialities, for example Mustang has fire and Armstrong has a highly physical type. It would be hard even for a good alchemist to learn multiple specialities (although scientists can know many things they can't learn every single subject in great detail), but theoretically a good alchemist could do it. As for Mustang transmuting the wall, while I haven't seen Brotherhood yet that sounds like something really basic that the average alchemist could do.