Question How would I add rats to the petshop?

and set them so you could buy them and have them or one as pets?

It would probably depend on what sort of petshop you are using. Is it an existing TBA petshop? Which vnums does it use? Is it possibly the triggered shop?

it's the one in the business district near the wharf and docks.

Did you read help petshops? And if the petshop you're thinking of is 31523, have you noticed in the rlist
24) [31523] The Pet Shop
25) [31524] PETSHOP STOREROOM