Can I specify the form to use for tickets that are created by email?
Yes, you can specify the ticket form to use for any tickets created by email.
To set ONE form as the default form, you can do so in for
Admin Settings -> Ticket Forms and then look for the Default Form box. The default form will be the one that will be used by all incoming tickets created by email. You can change the default form by clicking on the pencil icon.
To specify a default form for each queue, go to
Admin Settings -> Queues and Emails. Click on the queue and from here you can choose the default form to use for each queue.