To Gmod, all players not assigned a group are automatically "user", so that rules out "lower" than "user" without you manually/programmatically intervening.
Then you're running into the same topic as we have regarding 'why it's stupid to have a group higher than superadmin', but instead, with a 'lower than user'.
Same reasons. Gmod code by default without overriding only knows 3 groups.
As for inheritance, you do not have to inherit any group in ULib.
You can have a totally separate inherit from nothing group (which, it would automatically inherit from user, that's how Gmod works)
In that case however, you'd have to manually specify any ulx commands / access in other scripts you wanted that particular group to have (that wasn't already in "user")