If I have an exisitng 200a 240v 3 ph 65kaic panel and I wanted to add a 100a subpanel off the 200a do i need to stay at 65Kaic or better or can I go down 22kaic.
The 65 kaic rating could be in excess of what was needed when it was originally built. If the available fault current at the existing panel is much less than 65 kA, your KAIC rating only has to be as large as necessary for the fault current at the location of the panel. The fact that it is 65 KAIC doesn't mean your components need to match it.
Fault current at a transformer secondary, worst case scenario, is amps associated with the KVA rating divided by impedance as a decimal. After a feeder length, it also depends on the impedance of the conductor paths to the location in question, that depends on length, size, conductor material, and raceway magnetic properties if applicable. Explore a fault current calculator, or short circuit current calculator, to get a realistic value.
One way to get around this, is to take credit for equipment combinations with listed series ratings. Some breakers are rated together as a series combination, so that the upstream breaker can protect the downstream breaker, even if the available fault current at the downstream breaker exceeds its own rating. It is also common that certain breaker models are listed for series-rating in combination with certain fuse classes, such that the upstream fuse protects the downstream breaker. This allows for reduction of the KAIC rating of the subpanel breakers. Again, it has to be a specific combination that is listed as series rated and documented by the breaker manufacturer, in order for the breaker to be used where it otherwise would need a higher KAIC rating..