Nice to meet you cur_dog.
I have a knight .50 stainless Disc Elite. I use 250 grain Barnes Expanders on top of 100 grains of American Pioneer powder (two sticks) for my hunting load.
http://www.americanpioneerpowder.com/Bullets are supposed to be easier to push with the A.P.P, it's still pretty hard though. I think thats just one of the things you have to deal with to have an accurate smokepole. This load is very accurate in my rifle.
Then again so is everything else I tried. 300 grain SST'S , 240 grain XTP's .
About the mixing of pellets YES you can mix any configuration you like as long as you don't mix powder types. Just stay below the max charge recommended by Knight for your rifle.
The Disc Elite is a sweet rifle you just can't beat it.
Nice looking deer there.
Here is the link from my first range test with my Elite.
http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?p=461944&highlight=#461944