For better gaming? Get the 2TB SSHD. An SSD doesn't matter too much for gaming as its not constantly reading or writing anything-you download the game (write) then you are reading the preinstalled game. Faster read spead wont net you any better performance, I've done tests between SSD's and SSHD's to prove it. Get the 2TB so you can fit more games on there and you'll be happy.
Thats a silly question, as drives don't care what other drives are connected. All that matters is your PSU has enough connectors for more drives!
so yes, it will be "compatible" with your SSHD.