Can Sishtachara make caveats to Smriti?

So I know how Smritis can make exceptions to general injunction in the Veda. For example, the Vedas command that we should not kill any beings. But the Smritis make an exception for Kshatriyas and for self defense. This is Visesha Dharma and Apad Dharma. (For a similar reason the Smriti custom of Sati doesn’t violate the general Vedic prohibition of Atmahatya) My question is that for general Smriti declarations, can Sishtachara (practice of the cultured people) make similar caveats, as permissions or prohibitions?