It's a must that all employees follow the rules of the company, (won't it - don't they - mustn't it - mustn't they)? I think the answer should be "isn't it," as you said that the question tag depends on the main clause.
Sender
Ahmed
Position
Student
You are correct. The main clause is It's a must, and the correct tag is isn't it. The tag is normally based on the main clause unless the main clause expresses the speaker's opinion (I think, I believe, etc.).
It's a must that all employees follow the rules of the company, isn't it?
All employees must follow the rules of the company, mustn't they?

This use of must as a noun is rather informal. I don't believe must has been presented as a noun in the textbook.