SSD adapter for optical drive
Regarding SATA I, II and III and getting the best speeds speeds — it really depends on what specification your machine is, and what adapter you get.

I would suggest you install the SSD in place of your original HDD, that way you don't lose out on the SSD's potential because of an adapter:


MacBook Pros from Early 2011 and onwards do have SATA III (6 Gbit/s) connectors for their main HDD compartment.
But ones from Mid 2010 and before have SATA II (3 Gbit/s).


In either case, I would install the SSD there, and then install the HDD in the optical bay, because:


From what I know, newer MacBook Pros support SATA III in their SuperDrive bay — so to take advantage of it, you'll need to get an optical bay adapter bracket that supports it too.
MacBook Pros from around 2011 (and older) have SATA II in the optical bay.


To find out what you have, go to Spotlight (the magnifying glass in the top-right of your screen), and type "System Information", open that app, go to Hardware &gt; Serial-ATA &gt; Link Speed and it should say there.