A rapheal cat would depend on how big it is and how big the pickerel is, if the rapheal is to big to really swallow the pickerel might leave it alone if he is well fed.
That shouldn't be a prob for now, a 3" pickerel is not going to tackle a fish that big. Try to make sure the pickerels food is small and easily swallowed. If the pickerel grows too fast it might eventually try to eat the catfish with bad results.