Hi Tom, I can't speak directly about WMP9, as I'm running WMP11, but some programs give you the option whilst installing, to decide which file types you want to associate, or make default, with that player.
And you do also have the option usually, of setting file associations from within a program. WMP11 does that with Tools / Options / File Types, and you check or uncheck the boxes to whatever suits you.
You can, as far as I'm aware, have as many media players installed on your pc as you want, all operational, without them conflicting with each other, as long as you don't run them all at the same time. There may be exceptions to that, but I'm sure Spider isn't one of them.
If you want WMP as your main player, just check all the boxes, or uncheck all the file association check boxes in Spider, if it has that option.
That won't stop Spider, or other programs from playing those files, it just means that if you double click a media file itself, your default player will automatically play it.
I can't see shortcut keys having a problem, even if the keys are duplicated on diferent players, as long as you're not running them at the same time.
I've deliberately ignored the "Start / Set Program Access and Default" route, as whatever I set in there, it never stays the way I set it.
I hope this is what you meant tom, and answers your questions, if not, please post back.
Edit: I've never used any extra plug-ins, so haven't any suggestions. Sorry.