Your questions are probably a little beyond this forum's remit, but I'll try.
You should probably talk to Mr.Mac about the computer issues, but my understanding is that you need to use an aggregate device if you have more that one external equipment connecting audio to your computer.So, if you connected audio from both your Apogee and H8000 to the computer you would need an aggregate device, but if, for example, your H8000 was only connected externally and not to the computer, you would not.
As you mention, you would probably want to use the Apogee as the clock master, and sync the H8000 to it using wordclock or optical – it is best to avoid using FireWire as a clock source.
