Is it possible to change the "priority" of the default and addon AI so that FS fills up the parking spaces with addon AI aircraft  first, and then puts default in any remaining spaces.

This would save me from having to remove the default AI which could leave some airports empty - so I've heard...

To give priority would involve editing both airport parking and aircraft.cfg's. And then it wouldn't be guaranteed.

Here is a neat little package called FS2004 International Airline AI traffic Package.

It replaces the default commercial ai with rw liveries, but leaves the ga traffic as is. Although not on rw schedules, it beat the default. Compile requires TTool, which I think is included.

The commercial ai aircraft replacements include animated ground support equipment when parked.

Another option would be the PAI Commercial Traffic remover.

Dave