Can kayastha be of kashyapa gotra?

Yes, Kayasth boy and girl with Kashyap Gotra can marry.

Two gotras having a common pravara are not allowed to marry. For instance those of theKashyapa and Shandilya gotras cannot intermarry because they share the same sage, Asita, in their pravaras.

Refer: http://www.indiancultureonline.com/details/Gotra.html