Model within a model

That is not a good practice. However, to get past your current situation, you can do lazy loading of the needed models. That is, you do not load them on the needing models' instantiation. You can do something like this.

PHP Code:

class Settings_model extends \App\Models\BaseModel
{
    public $users;
    public function setUserModel(User_model $users)
    {
        $this->users = $users;
        return $this;
    }
}

// When creating the models, you can this;
$users = model('Users_model');
$settings = model('Settings_model');

// now your settings model has an instance of users model
// you can do the same with users
// but beware that this may cause memory leaks
$settings->setUserModel($users);