My concern is that the FlightXML2 documentation is misleading (see below). It has no distinction between commercial and general aviation and can be interpreted as using either the tail number or ICAO flight number interchangeably… but from your response, that’s not the case, right?
" InFlightInfo looks up a specific tail number (e.g., N12345 ) or ICAO airline and flight number (e.g., SWA2558 ) and returns current position/direction/speed information. It is only useful for currently airborne flights within approximately the last 24 hours. Codeshares and alternate idents are automatically searched. "