Can I call an existing stylesheet for my form? Or a new one that I've created?

Yes, you can.

To do so, echo the link rel inclusion as a BEFORE FORM piece.
Form Properties > Advanced Tab > More Options Link > Form Pieces
Then in the BEFORE FORM section, click on the CUSTOM button and add the following code:

JFactory::getDocument()->addStyleSheet( JURI::root() . 'PATH-TOCSS'); 

Then click on the SAVE icon at the bottom of that screen, and then, once back at the form properties screen, click on the SAVE icon in the upper left-hand corner of the screen.