Just for testing's sake, I installed CustomPress with Twenty Eleven theme (only other active plugin is WPMU DEV Dashboard).
I added a new CPT and named it 'post' (a WP default post type).
To my surprise, I didn't get an error message (like that post type already existed).
So I added a custom field and it showed up in the default 'Add New Post' screen.
Then I made that custom field required, and it worked as you'd expect, saving a draft but saying I cannot publish until I enter something in that custom field.
However, I went to uncheck the box in CustomPress for the 'editor' feature. I expected it to work like this:
no WYSIWYG box (since I unchecked it, and verified it's still unchecked)
enter req'd custom field
I'm looking for some confirmation that 'post' and 'page' and other already-registered post types are meant to be allowed in the CustomPress plugin's settings.
Secondly, why doesn't unchecking the box for 'editor' take away the Add New Post WYSIWYG editor?