The Cat filter will not fit the AirDog. What he meant was what I also did, added a Cat 2 micron filter and head to his truck. I put mine between the stock filter housing and the CP3.
The AirDog has a filter and a strainer, but no water separator. Yes, there is a filter with a drain on the bottom and that is simply a strainer meant to keep capture particles big enough to damage the AirDog pump. If you want a water separator then you would need to add it after the tank and before the AirDog.
Godspeed,
Trent