I'm not sure if this is available or if it is a future request. Is it possible to choose a product category so that it appears under swap product for a specific product page. Therefore, when I click on customize for that product it opens on that product but if I click on "swap product" then I see all products in the category and I can choose any product from that category?

Please submit your request under the FPD "Feature Requests" section. 

https://support.fancyproductdesigner.com/support/discussions/forums/5000217737