Welcome, Spidey, both to the boards and to the wonderful world of customizing!
I agree with Ryan's answers, but have just a few things to add. First of all, I found out the hard way that Aurra Sing's ball-joinged arms do not pop all the way out of the torso -- the shoulder pegs stayed right where they were, since they were made of a harder plastic. So, just by way of warning, Aurra Sing is one exception to that rule!
Also, as far as checking figure compatibility, I know that FFURG.com has some lists of that sort, though they're by no means exhaustive. But Ryan's right that most 1:18 heads out there can be used, with some alteration of course. I should think the limbs from the XD figures would work out too, since they seemed to be attached to the figure in the same way.
Hope that helps! Happy customizing, and keep in touch!
... the Findswoman