mines are allready capped at 50 afaik.
but there's little point in this suggestion imo;
- scouts provide sight, can carry mines, and take a lot less space (high lvl ones are faster as well)
- normal bomber pilots can load skytraine & bombers, so why load less effective skytrains with mines over bombers, if you're loading any (because scouts would be better at that).
you want to make the paratrooper/landingforce sailor more usefull, but it doesnt really have anything to do with flying skytrains, thats a bomber pilot ability.
** not sure if all this is still up to date after the mini HA - skytrain patch, but i think it is.