Theoretical: Should models be subclasses of AR or mix it

That sounds like single inheritance? Except without a type field?

I actually could have used single table inheritance in my code, but thought
the two relations I wanted to store, "carpools" and "carpool notification
erquests", were distinct enough that they belonged in different schemas.
Also, I wanted to avoid having NULL fields. Here's the relevant migration:

create_table "carpool_notification_requests" do |t|
  t.column "email", :string, :default => "", :null => false
  t.column "starting_city_id", :integer, :default => 0, :null => false
  t.column "destination_city_id", :integer, :default => 0, :null => false
end
  
create_table "carpools" do |t|
  t.column "starting_city_id", :integer, :limit => 10, :default => 0, :null =>
false
  t.column "destination_city_id", :integer, :limit => 10, :default => 0, :null
=> false
  t.column "leave_at", :time, :null => false
  t.column "return_at", :time, :null => false
  t.column "email", :string, :limit => 70, :null => false
  t.column "notes", :text
end

And the subclassing model:
class CarpoolNotificationRequest < Carpool
  set_table_name 'carpool_notification_requests'

  # Carpool has validation which don't make sense here
  def validate
  end

end

The reason I subclassed Carpool was to inherit its belongs_to relations and
some of its methods.

I hope this helps!