An AOC operation with less than 19 seats is an Air Taxi operation unless it is scheduled routes. That is the definintion.
From Regulation 965/2012 as amended, Article 2:
’air taxi operation' means, for the purpose of flight time and duty time limitations, a nonscheduled on demand commercial air transport operation with an aeroplane with a maximum operational passenger seating configuration ('MOPSC') of 19 or less;
So yes, there are different sets of rules for different AOC operators.