Good choice of DIO card, but be aware of limitations too, in terms of the number of dynamic waveforms capable channels required, not all channels are waveform or HW timed capable.
Yes, VOL, VOH levels are different for PXI/PCI and PCIe.
In the specifications, worst case voltage levels are defined,
This says that each DO channel source/sink 24mA, for example, your external device is a relay that consumes 24mA, in this case, when you source Logic High, it will be at least 3.4V guaranteed (but it will still try to source 5V) and similarly if there is a load sinking into the DO, then at 24mA it will be able to maintain the voltage under 0.78V.
Do you know what kind of current does your 'external device' will consume from these DOs?