p986s- Short answer: all PCs can use whatever size mounting plate. As long as it has the right threaded mounting post, it'll work. 4, 5, 6" will all work.
Haven't tried my flex plate yet, but I sure HOPE it works OK! It might work better/worse (i.e., with more or less vibration) with one counterweight or the other, though. Just looking at/holding it, I'd say it'll work best with the 6" counterweight.
It's best to have everything JUST RIGHT, but you can fudge this a lot, too. I myself like to have different size mounting plates (and matching counter weights) handy, they're pretty affordable.
BTW, *I* would take the current discourses on "German paint" with a little grain of salt. I seem to recall that BMW paint IS a little softer than some others (haven't owned a Bimmer for quite a while, forget what it was like..) but people have got GREAT results on them with all kinds of pads. And that Green Giant pad (another new purchase I haven't gotten around to putting to use

) is gonna be *SO* mild that you'd better have a nearly perfect finish (as in, NO flaws that you can see with the naked eye) before you use it. *I* don't even really plan on using it on basecoat/clearcoat cars, but rather on soft single stage lacquer. The other thick PC pads I've used were so mild I had forgotten all about them until the recent Green Giant marketing jogged my memory.
If *I* were doing a BMW, I'd just use the regular Griot's, Lake, or Meguiar's pads, but that's just me. The flex mounting plate seems like a good idea though.