Is there a way to add a button to my form that does not submit the form but performs a mini validation and adds more inputs to the form
You can use #limit_validation_errors to limit the fields validated when a form is submitted with the button you add.
If you set $form_state['rebuild'] in the validation handler you add for the button the form will be rendered again instead of being submitted.
Then all you have to do it add some logic to the form that will display the extra fields. This can be done with the data in the $form_state.