The rank of your gals is based almost entirely on crew size. I just can't see a well-armed crew surrendering to a bunch of poorly armed, unskilled gals.
Hmm.. I though it has some other requirements for ranks to be awarded other than just number of hands, well never had too many hands before some of them were almost maxed out.
If that is the case then the most logical, but not sure if possible, solution would be to rely on commendations your hands earn, like enemy might not be too keen to face hand know as Scourge of Europe, Bane of Academy and Master of Sledgehammer. With various base points for each title (Person of Mass Destruction being scarier than Patient) multiplied by rank - sum it up and compare to enemy level for encounter or somehow sum ranks of individual enemies to account for how many there are. This seems to me as most logical way to check "reputation" and its potential effect on the enemy.
Instant surrender seems a bit OP, but lowering starting morale by up to, lets say, 50% of its original value for side being in disadvantage could result in fast cleanup operations, or on the receiving side, taking cover for a few turns and slowly advancing to let morale regen