Most players tell you to do as they say that they do, and specialise in one type of troop.
Though there are potential drawbacks to this strategy ,there are advantages to be had by doing so. because you cannot acquire sufficient skills to accommodate every class.
You will have already noticed the obvious , the faster troops generally carry less, and those that move slowest carry more.
There are others things to decide when and if you specialise .
Suggestion: In the Valkyrie, think long and hard about what troops you want to upgrade first. I didn't know what I was doing when I set it up and made scouts my 6th level warrior, meaning I'm 99% sure I'll never be a good scout because it'll cost waaaaaaayyyyy too much to upgrade.
A lot of people (like myself) chose melee first because their upgrades are first in the T6 and T7 trees of the Oracle.
I have Killer class as my main with a secondary of Siege. They compliment each other. As my Killers are "maxed" for now as well as my Siege I've started Melee as the third on the list. I also have been putting a few and I mean very few upgrades on Horsemen as I use those every now and then for the speed to tiles. It will be awhile before I can do anymore upgrades on either Killer or Siege so will continue to up my Melee and possibly do a few for Calvary and Scouts. I've been compensating for my Scouts being so weak by training a LOT of them lol mostly t-1's and t-6's.