I'd appreciate any advice.
I have a Bushmaster Carbon 15 with Bushnell 1X30 RedDot sight and Firefield QuadRail.
I seem to be one of the few who actually likes the Bushnell RedDot, but I'd like to add my Magpul Gen 2 MBUS front and rear sights as well.
I'd like advice on the best way to accomplish this: Raise the RedDot (if so, what type of mount?) or use picatinny-mounted riser for the MBUS (front and rear, or rear only, and what size to get a good co-witness)?
Much obliged for any and all advice anyone might offer!

A picture of your current setup would help, but you can probably just mount your sights to the top of the receiver and quadrail, then get a riser for the red dot. I don't know anything about that quadrail - if it's free floating and extends out beyond your gas block you're good to go; but if it's non-free floating and stops short of your gas block then it won't be ideal. There are lots of risers for your red dot out there, this is just one example:

http://www.midwayusa.com/product/878944/weaver-single-rail-system-scope-base-ar-15-flat-top-matte