I would first recommend you review this: OWC Data Doubler note the Red text at the bottom. It makes no difference who's optical drive carrier you use if you have the system that has an issue with the optical drives SATA port.
As to where to place your SSD I would recommend it put it in the optical drive carrier as your systems HD crash guard protection is only located in the HD bay so moving the drive over to the optical carrier will put it at risk for damage if you bang the system while the system is running.
As to the size the carrier can support the limit is the height of the drive (Z). The limit is 9.5 mm. The note is a bit dated as many of the 1 TB drives were higher. If you review the spec sheet of the drive you are looking at you want to make sure its not too high no matter which bay you put it in. Both are limited to 9.5 mm.
So to review things here:
- Leave the HD drive in its current location otherwise your drive could fail
- If you upgrade it it still needs to be no higher than 9.5 mm
- Adding a SSD in place of the optical drive maybe tricky! Depending on your exact model review the OWC note.
- If the best you can put in is a SATA II (3.0 Gb/s) drive make sure you get a fixed SATA II drive, don't get an auto sense drive (one that can run in either SATA II or SATA III systems) as it won't work either.
Personally, I would recommend you go with a SSHD like the Seagate Laptop SSHD as your HD replacement and leave the optical drive alone. That way you get the bang of the deeper SSD cache in the drive and yet have the larger traditional storage it offers. Here's the IFIXIT kit: 1 TB SSD Hybrid 2.5" Hard Drive - Upgrade Kit which has everything you need.