DX says it's 12.5" stock. it's actually a bit longer, around 12.7" or so, eye to eye. if you get the 12.5" Arnott 440's they will lower your bike a tiny bit (more than 0.2 inches though, given the geometry). 13" will raise it, of course. most with the DX have gone with the 12.5, unless you really want to lower it a bunch and go with 12"
I LOVE the combo of Penske & AK-20. Traxxion does work for (and with) Penske, so they're familiar with my rear setup.
You spec the pre-load when they're built - your weight, with standard load (saddlebags, etc.) and whether you ride 2 up often.
Mine was spec'd 2-up occasionally, and my damping/rebound is soft for general beating around, and a couple of clicks harder when I'm out riding with my ricer friends. I really need to get down to the next open track day at Homestead, to get all my setting "ridden out" and documented (and I doubt my old race-leather are gonna fit anymore).
Two up is barely noticeable in difference, though the balance of the bike is pretty fine with my setup, suspension feedback is much more "detailed" (for lack of a better word) than stock. You can tell if your passenger isn't far enough forward. It doesn't wallow or anything with a passenger, but you can feel the center of gravity shift towards the rear somewhat. As long as they're positioned properly (nipples digging into my shoulderblades), they're barely noticeable (except when their nipples get hard). And my tastes don't run towards the "fatty" end of women. As far as the Penske's go, a few clicks on the damping should smooth out any additional rebound caused by a passenger.
This ain't a dresser anyway, so the "leaning back on the backrest like you're on a couch" thing, is going to throw the balance off with ANY suspension.
Suspension choices are really dependent on riding style. If you're setting up as a "cruiser". one is pretty much as good as another (with air ride probably being best for "cushiness"). I'm setup for the "sport" end of the sport/cruiser genre. Tire chatter while hard cornering is unacceptable to me. This setup is about as close to ricer/sportbike suspension as I'm gonna get, and I've been extremely happy with it so far.