if you use a brifter for the rear, you'll be using a center-pull canti or one of the avid bb5/7 "road" discs. If you're buying a salsa, it's disc-only. If you're buying a gunnar, it's possible to get canti-posts, but it's an extra $75. BTW, the gunnar is still made in the usa, whereas the salsa is made in taiwan, which is why dude at soulcraft makes soulcrafts now, but i'm digressing. I'm just saying, if the country of origin means anything to ya, grab the gunnar.
I run a mountain bike with thumbies and some "traditional" long-pull levers. Works really well on my "alt.bars" (titec j-bar) because i can cram them almost anywhere i want them. I got a buddy with the same bars, but he's running the old deerhead xt thumbshifters, because these have a split-clamp, and can be put virtually anywhere.
Best of luck to ya; enjoy the build. The main difference, in terms of compatibility, between the slx and the xt is that the slx offers triggers that are specific to a double or triple crank. So, if you're running a triple crank, don't get the double shifters. But, you'd probably figure that out on your own. otherwise, things'll interchange very nicely.
hth,
-rob