That's right, currently the admin is just like any other group, with the added ability to manage groups and users... but this makes me realize I should automatically grant them full access, since they can anyway log in and do it themselves. So what I'll do is that if "Is Admin" is checked, the other two checkboxes would hide, and be replaced with an "Admin has full access" kind of note.
The idea is indeed to extend this to other permissions like you mention. But gotta wait for a future release 'cause I'm almost done with 1.1