All ok, but I think the last two sound better like this: I just got my hair cut I've just gotten my hair cut

No, they are a little different in meaning. If you say "I've just eaten breakfast" it is as though you are speaking to someone right now about something that you did recently. If you say "I just had breakfast.", then you mean that you had eaten breakfast that was a recent event from the viewpoint of a given time in the past. Of course, this time in the past could have been anytime. It could have been two seconds ago, in which case the two sentences do have the same meaning. You could also say " I just had breakfast, when there was a knock on the door". This sentence could have taken place 5 minutes ago or 5 years ago. The reference time was the moment when a knock on the door was heard.