Is a stool sample necessary before getting a colonoscopy?

No. you mean for blood? it is an alternative method for colon cancer screening, but not, it is not needed if you're getting the colonoscopy anyway.
No, Not necessarily. Unless the doctor is debating whether to do a Colonoscopy or not. For routine screening,, colonoscopy is done as a stand alone test for early detection of Polyps and colon Cancer.