the shop will set the din for you, they should have a form for you to fill out that gets your height, weight, ability etc and then they'll work it out from there. 6 would probably seem about right though, maybe a bit less if you're not a confident skier
the plastic pieces? I'm guessing they're raisers, if you get the px12 jib they're supposed to absorb some shock. also they lift your boot higher off the ski so if you're carving a hard turn your boot won't dig into the snow.
the stock brake for px12s is 80 or 85mm I think. it should be fine for the fujas. the next size brake up is like 113mm which would look redic and probably dig into the snow when turning. if they don't fit it shouldn't be hard to bend them a tiny bit to get them to fit. to put it into perspective, the stock brakes nearly worked on my anthems (93mm)
just have a look around online, backcountry.com might be a good place to check