how about not a percentage of crew but a steady increase in quality of the marines themselves
for instance common marines as they are now
marine percentage 0.2
marine level 1
marine melee modifier 1
firepower modifier 0.1
common marines in my suggestion
marines percentage 0.1
marine level 2
marine melee modifier 2
firepower modifier 0.2
and so on and so forth all the way up to exceptional that way marines is a viable option for all ships and not just for trade capturing or just one guy to board in a group.