He confirmed that yes, the AJA io XT does currently work with FCP 7 and Yosmite, but that it may not in the future:
"I spoke with our tech support group, and they tell me that our engineering team has been able (thus far) to retain full functionality within FCP7 with IoXT. No real limitations exist that they know of at this point. Of course, we cannot guarantee continued support indefinitely, as Apple is no longer updating FCP7, but for now, it works wonderfully. Also, once you have the Io XT, tech support is well versed in assisting you with any issues you run in to..."