Kydex/Concealex will not scratch any knife--on its own. When a knife gets scratched by a kydex sheath, it's because crud (that's harder than the knife steel) got stuck in the sheath. Sheaths can be designed to overcome this, though. One of the most important of these features, IMHO, is the ability to clean your sheath. This is easiest accomplished when all the fasteners on the sheath can be easily removed, but can also be done if the sheath just has a drainhole.
If Eric and David don't over a drop-leg option, check out GPB's (Frank Butterworth IIRC) sheaths; he makes a great drop leg, and I've seen it several times on Mojos.