When my Symphony I/O is in USB Audio Interface Mode, can I sync to external devices?

No, When the audio interface mode is set to USB Audio, the Symphony I/O operates only on internal clock.

If you wish to use a device via SPDIF, AES or ADAT connections you must clock them either via Word Clock Out of Symphony I/O, or SPDIF, AES or ADAT Outs, setting the other device to External.