I see that the USB-6251 can run in Full-Speed or High-Speed mode. Is this something I can configure in the NI-DAQmx API, or is this automatically decided by the USB host? Haven't been able to find anything in the reference pages one way or another.
The speed that the spec sheet is referring to is the speed of your USB bus. If you connect to a High-Speed USB bus the device will operate at High-Speed. However, if you connect to a Full-Speed bus, the device will be limited to the max speed of the Full-Speed bus.
It is not a configurable setting because it is a function of the USB hardware you are connected through.