FSX and FS9 share a lot of things, but FSX adds addtional content and chnages some things, so whether or not a scenery or plane for FS9 will work in FSX as well is a matter of trying out the individual software in question. chances are that in most cases it will work.
If you are into soaring, consider "Condor". It'S the best thing you can get on PC for soaring. I even had a review of it in this forum some time ago. It is not perfect, but simply better than any other PC approach to soaring. But it is difficult, and probabyl only for the specialised entzhusiast. Most users of Condor are real world soaring pilots, I got the impression. but maybe that is just me.
http://www.subsim.com/radioroom/showthread.php?t=129997