Only role that really matters if it goes down is the pdc emulator, the rest don't really matter for a period. Its fine to put them all together if you have to.
There was a good reason to separate a role pre 2003 sp(1? maybe 2 cant remember) from a global catalog, but it no longer matters.
They can be moved with your ad tools, do a web search on moving fsmo roles and you will get a heap of step by step. Only catch is schema you need to register a module to have it appear, you also need to be schema admin and enterprise admin for moving various roles.