Can you use watercolor paint for washes and weathering?

Definitely.

Works best if you add a drop or two of dishwasing liquid as a surfactant. (Non anti-bacterial is best; some of those have an additive that can clot the paint.)

Yes you can. But always practice a new to you technique on a paint mule/glue bomb/etc first, not something you care about.

Key point is to protect the underlying finish, especially acrylics, with a clear coat, as excess water could damage or soften the acrylic finish.

Historical consensus is to use water colour/acrylics over enamel finishes or enamel weathering finishes over acrylics.

I tried it recently and was not happy with the result (did not use soap).  Next time I try it I will use drop of soap.