Can I work with just mandala designs on Adobe Stock portfolio?

Mandalas appeal to a very narrow group of buyers. If that's what you want to do, fine.  But don't be discouraged if sales are slow. 

Being a successful Stock Contributor requires a diverse portfolio of many different subjects. Ideally, you should submit artwork that appeals to a wide range of customers from various regions, cultures and interests. Cast a wide net to catch more fish. 

There is no problem with specializing in a particular design motif, though that will probably limit your sales. Diversity is often cited as the way to ensure a steady level of sales.

No, but you probably won't get much sales, even if Adobe accepts the files.

No, you should be okay. But, searching for Mandala I see 1.8 million entries. Do not expect a high acceptance rate and do not expect a lot of sales.