He's right.
'superadmin' is considered a root or all-access group by gmod.
While ULX will function properly with your custom above-superadmin groups, we can make no guarantee that you will not experience issues with other addons or gamemodes.
For more information about this, please refer to a post I wrote -
Here's the linkAs for your question on targeting, please refer to this FAQ on targeting.
Here is a linkAs for permissions. The way I do it is to start at the lowest group and assign all permissions you want them (and all higher) to have. When you have a group inherit another group they receive all permissions from that group. Then just work your way up the chain.
Example:
user - assign all permissions you want EVERYONE to have.
donator - inherit user, assign permissions you want donators and above to have.
moderator - inherit donator, assign permissions you want moderators and above to have.
etc.
etc.
Please take a look at those links and if you have any other specific questions on how to get yourself started, please don't hesitate to ask.