Having work experience in one category is it a must that one should work with the same category, if one is willing to explore and wants to learn the trade of different categories is it ok?

Yes definitely we should have to give chance to show its talent.

One has to decide between Specialist in one category  or a Generalist with experience in few/  multiple categories.  Both have their Prons and Cons. 

- As a Specialist, you focus on your domain, but limit the opportunities

- As a generalist, you can expand your domain by working / managing various categories and expand opportunities.

Note: As a generalist, do select the categories  where your interest lies. If you enjoy and like your categories  you will do justice to the role and make the business profitable.

If you want to become a manager, especially in retail it is important you get knowldge in all the categories.

as long it's your passionate categories you'll success in managing it. again knowing your products is the main key to success , just monitor number will never lead to anything!