Can I pair other Bluetooth headphones/speakers to the Harmony system?

No, the Harmony uses radio frequency, so the transmitter is not Bluetooth. Only Harmony speakers, Quartet-RX Headphones, and Duet-RX Headphones can be added to the system due to private protocol. You can add up to 100 Harmony speakers to your setup while maintaining low latency. For more details refer to page 13 of the user manual.