-Could you please propose a solution to create an advanced interface which contains jQuery script, 2 gridview and create a custom action for dropdownlist using nintex form?
-Is Nintex related to list forms. Can we use nintex form to create a separate interface (not related to a SP list).
- jQuery can be added into the forms, creating dropdown lists is quite simple to do (drag & drop onto the form, and connect it to the column or type in the actions). Not sure what you mean by "2 gridview"?
- Nintex forms can be created from a list or at the site level. Just because you create it on a list, though, it doesn't mean you need to keep those fields. You could remove all of the fields from the form and make your own - to start an external workflow, for example.
More details are likely required for your query! It sounds like you have some ideas about what you want to achieve.
This is more than what you need, but shows how you can add a dropdown and also jQuery - in this case, they are connected, and the writer is creating a cascading dropdown.