Can I use the Futaba FASST R/C and XBee with the same frequency (2.4 GHz) without interfering each other? Thanks alot !

Plan on there being interference as long as you are using the same physical frequency. The Xbee is designed with built in collision avoidance/recovery. Increasing the RR value will cause the radio to increase the MAC level retries. Remember that the MAC level retry is 3 retries minimum. If you are having a lot of issues try a different channel…