The amount of ai traffic an airport can handle is mainly dependant on the amount of parking available.
Afcads will generally improve ai traffic by providing more parking than the default airport layout so the airport can handle more ai traffic .
There are basically 2 types of Afcad design.
1-Afcads designed to work with the default airport scenery layout. These generally add parking (and other improvements) to allow more ai traffic.
2-Afcads designed to work with a specific 'custom' (more realistic) airport scenery layout. These Afcads are generally included with the 'custom' airport scenery download.
The file description will normally state if the afcad is for default or a specific 'custom' airport scenery.
Where to add the files??.. That's a matter of personal preference. Any active scenery area will do. In my case.....
If it's a stand alone afcad, for default airport scenery, I have an active scenery area named 'Afcads' and place the file there.
If it's for a 'custom' airport I place it with the rest of the files for that 'custom' airport scenery.
A list of afcads??.. not that I know of, though this site may help in your search-http://walhalla.mine.nu/fs2004.php
Or search your favorite download sites for the ICAO or airport name.
At that time  the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.