Compressed loads are not a factor with Varget but you always want to use a starting load and work your way up, usually you will find the best accuracy before you reach max. Since Varget is relativley large extruded granules, and the .223-5.56 case is small you find that you have a compressed load very soon.
Match primers could make a difference if you are shooting a custom chambered rifle but for an off the shelf rifle the regular primers should give all the accuracy you need.
Most factory 1in9 twist bolt rifles are produced to shoot every thing from 40 grs up to about 70 grains, which is why they twist with 1in9, they want the rifle to shoot about any type bullet right off the shelf. A rule of thumb; if you don't know for sure, over twist for weight-length.
It is the length not the weight that governs twist, and longer bullets that usually weigh more than shorter bullets require more twist to stabilize.
Mixed brass, you want to segregate military brass from your oridnary brass, the military brass is thicker walled and has less volume so it reaches max pressure with the same powder weight before the ordinary. It's not a problem you just have to work the load up for the military brass and don't mix the two.
If you are serious about reloading spend some bucks on reloading manuals, not only do they give safe reloading data but also a lot of knowledge. Read as much as you wallet allows, you will allways resort to you reloading library on questions, it's not to early to start.
Vmax's shoot great in my Savage 1in9 twist but my best was with the longer 69gr. SMK's .