Right johnny..typically in a 16 ch sum you'll have 4 monos..but that's not a rule..
You could have all ch switchable to mono..
My set-up will end to have 4 monos(kick-snare-vox-bass)..
Out of the sum i'd get into a gentle comp and gentle eq,and then back in the coverters..
If you want to give an extra bus comp to all drum-set,do it (in digital) before to go to the sum..
-very important-apply those gentle analog bus processing before you start mixing.
They have to be part of what you doing since the beginning.crucial.
Last but not least...passive summing needs amplification to survive...i beg you know that.