Can a person enroll into a full time course(masters),when the C.O.S. from F2 - H4 is pending?
The international student advisor says that you can enroll for one course alone and not more than that until the c.o.s. is approved.Is it true?

Correct... why don't you wait until you have a valid legal status? Or simply cancel your COS, and re-apply for F-1.