Kevin,
I asked John Norwood these same questions a few years ago. As you may know, Mr. Norwood worked for the Rio Grande from the '30's to the '70's, and has written a couple of books which I highly recommend on his experiences.
He said that there were a couple of factors in the Army's decision to get the K-28's, as opposed to the 27's or 36's; The Army figured the 450's and '60's were too small, (even though John said in reality K-27's and K-28's were about equal when it came to taking tonnage up a hill). And, while they wanted to get the K-36's to begin with, it didn't take long for the Army to realize that they were probably too heavy for the WP&YR. As a result, the Army didn't really put up a fight to get them. However, the K-28's were about right for weight and power.
Another thing to take into account are the rumors that the Grande saw an opportunity to phase out the narrow gauge by getting rid of as much motive-power as possible. I'll let the Rio Grande historians, (of which there are many...) debate that. For instance, I don't think the Rio Grande put up much in the way of a fight to keep any of the equipment that the Army wanted, not that it would have done much good anyway. I seem to remember hearing that it was a state-representative or someone of that nature who actually was the reason why the Army didn't get all of the '28's. I'd like to know more of the story on that...
And, contrary to popular belief, the '28's performed rather well on the White Pass. I've talked with quite a few shopmen and firemen who were there during the War and, to a man, they all say the 'Rio Grande Engines' did fine.
Hope this helps in some way!
Casey