Even without a house rule allowing crits when soak reduces damage to zero, I think the 'balance' solution here is a combination of pierce and crits. When you're applying these mechanics appropriately, the game works relatively well, and there's no need for house rules.

However, if you still *want* house rules, then use house rules.

One could give the shivs pierce conditionally.

Pierce 2-3 and a new rule called “Shank”: this weapon loses the pierce quality when used against a target wearing any kind of armor, or against droids.