Can I change / customize the HTML?
Yes, you can 'override' the view files. For example, you can copy 'packages/page_feedback/views/page_feedback/form/simple.php' to '/application/views/page_feedback/form/simple.php' and then modify its contents. Make sure you clear your cache if you don't see any changes, because the override paths can be cached depending on your cache settings.

What happens if multiple forms 'match' a page?
The first form that matches will be shown.