There may not be a way then for peons to be immune, and still allow for admins to overcome them without adding "overcomeimmunity" to the admin group, which then would of course allow admins to affect superadmins which you don't want.
Immunity wasn't written to be as diverse as the access you're trying to give 'peons', but really only meant to deal with superadmin and admin access mostly.
ULX by default has no way to allow peons as a group to only use commands on themselves individually.
If you give a peon "ulx god" or "ulx ragdoll" access, they can do anyone that doesn't have immunity.
However, with that being said, you may find MrPresident's release "
Selfcommands for ULX" handy. See our release section for other items. Remember, there's more than one page of additional content that can be used with ULib and/or ULX