>Am I less of a Catholic if I don’t wear one?
Not at all.
>My understanding is that I would need to be enrolled in a devotion by a priest, is that necessary as soon as I get it or could I wear one without talking to a priest immediately?
You can wear one without being enrolled but as I understand it you won't receive all the graces associated with it.
>is it ok to buy one from Etsy or Amazon?
>My confirmation Saint is St. Michael, could I wear the scapular of St. Michael?
I don't know about all the different scapulars aside from the brown and the green ones, so maybe someone else can help you there. I wear the brown one myself.