I have used skins all the way back to 2003 with no real issues. The biggest difference is there are fewer "special features" in the elderly skins that you would see in newer skins. If you look at some of my older reviews, you'll see I have shown off some of those elderly skins and mentioned in my comments what the pro's and cons of working with older skins can be .
As giz pointed out, there are a few glitches with some of the older skins, but for the most part there is no reason not to give ol' grampa a chance.
As far as vista skins go, there are a few that just won't work at all in XP, a good example of this is Kitty Malone's skins. If it says "Vista only" it really is. if you try to use it in xp, the graphics just won't mesh and the whole thing looks sloppy. I usually wait for her to make the xp version and am always pleased with the results.
I can't suggest what you should do, but I can tell you what I do. I try each and every skin, to see if there is an issue or not. You'd be surprised how many of the vista only skins will work on xp. You might lose some of the special features of the occasional skin, but I haven't found too many that I just couldn't live with despite those losses.
oh, and if you find an older skin that you can really show off to it's best advantage, please share with us! show us a screen shot of how you made it shine , let us in on your secret to making the best of skins so many have forgotten are there.