You can place almost any skill anywhere, with only restriction being some skills cannot be placed on left-mouse slot because they wouldn't work when bound to the same key as the "move" command (though hopefully this will change so we can bind move to left click and the #1 ability to a different key).
In-game you'll have to turn on "elective mode" in options to be able to pick any skills you want in any of the slots. The default mode is really just there for newbies to get an idea of what might make a good build. In reality, no builds I've made so far every followed that pattern - I've always picked a skill of a "mismatching type" for at least 1 skill slot, for all builds I've made for all classes. Even in the beta I've found myself taking some advantage of mixing the skills in a way Blizzard doesn't intend newbie players to be mixing.