Is it mandatory to enroll in a debt relief plan if I sign up for counseling?

No. You're not bound to sign up for a professional debt relief plan if you go for debt counseling. If you think the debt relief company is not a reliable one, you shouldn't enroll in their program. Moreover, you should ask any question that comes to your mind. Also, make sure you ask how long it'll take to get out of debt if you enroll in a debt relief plan offered by the debt relief company.

I fully agree with GOODNELLY., It's not necessary to go for a debt relief plan if you're not comfortable. You must verify all the aspects before choosing the right path.
But it needs to be considered that a credit counselor will determine a debt relief plan only after analyzing your financial situation. The final recommendation given by him will be totally based on your financial status and the best solution to reach that goal, which often is a debt relief plan.

No, but you should verify the credentials of the firm you're working with. And never make an payment upfront, thats against the law.