Can I use a custom page to forward to once someone signs up?
Absolutely, however this needs to be set up so that the list-database is your own custom list-database. If you'd like to use the SimplyCast CRM with your Signup Form you can; however, it will need to use the SimplyCast-provided forward page to ensure all of the post data is properly passed to the list. If you're not sure what any of this means, it is probably best to let SimplyCast handle form processing.
In order to have your own custom page to forward to after signup, the page you are using must handle all of the post data. We recommend only using this if you are an advanced user.