I agree with most of what's been said above...
The valve body is no smaller on a Cornet usually so I don't think that solves the problem.
I would say to hold the horn however you are comfortable! How you hold the horn should not adversly affect your playing.
As long as you can operate the 3thd valve slide your good.
I use my ring finger for what it's worth.
I have the opposite problem, as a 6'3" 340 Lb man I have large hands so I often find the valve block to be constrictive.
My .02
good luck!