I agree with Mordain Thade. As a companion to the rapier, it should function similarly to a rapier. A rapier does not have special disarm abilities, so why should the main gauche?
To keep it equal to the kukri functionally, (thus keeping it marital), I would substitute the 18-20 crit range with having it grant a +1 shield bonus in any round the user does not attack with the main gauche (maybe double it to +2 when fighting defensively/total defense. This makes sense since it is more of a defensive weapon than an offensive one.
Adding disarm abilities should, at least, make it exotic, if not magic.
Just my 2 cents.
