The Rules Upgrade (transcribed from Star Wars Adventure Journal, issue 11) that WEG released would probably have the most complete listing of what actually changed. The blurb at the top states:
Aside from the extensive physical makeover of the book, as well as the inclusion of a much needed index, there were only a few changes to the game system.
With which I am inclined to agree. The actual differences are small, mostly clarifications and errata.
I personally prefer the R&E over 2ed because of the additional clarifications. I wouldn't try to use them together, as they are not strictly 'compatible' ( less so than D&D 3e to 3.5, which is comparatively a much larger revision ) because most things that are changed are changed in small or subtle ways. As for using other 2ed source books, i.e. not the core book, I have yet to run into any problems with it, though I tweaked a couple of the powers to bring them more in line with the others.