Majority of your questions are answered in our FAQ (Frequently Asked Questions) section -
/index.php/board,30.0.htmlULX adduser is mostly permanent, so if it's not working for you, you likely have a conflicting admin mod. Since you mention owner, you likely have DarkRP, which has an admin mod that often conflicts with ULX. A FAQ for disabling the darkrp admin mod portion is included in our faq section.
If you decide to make an owner group, have it inherit superadmin, which is what Garry's mod expects to be the highest group.
I can't say I recommend actually creating any group "higher" than superadmin though.
See the 'can target, how it works now' FAQ for setting up your inheritance/can_target tree to prevent those 'under' you to perform functions on you.