Do I need to get separate SATA cables to upgrade my SSD to another SSD, and my optical drive to an HDD? I have a 13" mid 2012 MacBook Pro, and I want to upgrade the storage capabilities. Specifically, I want to replace my 125 GB SSD with a 250 GB SSD, and my optical drive with a 1 TB HDD.

The SSD inside my MacBook already has a cabinet, I assume. Hence, I do not need to get another cabinet for the new SSD. I can use the same SATA cable too, correct? However, I need a cabinet for the HDD, and I was thinking of getting an OptiBay for my unibody Mac. I will need a new SATA cable to connect the new HDD, though, right? Do I need some kind of special SATA cable, or will any do? How long does it need to be? About 10-15 cm?

As for tools, I think all I need is a regular X screwdriver and a Torx size 6.

Have I assessed my needs correctly? I want to have everything in order before I begin the procedure.
I made a similar modification to my 13" mid-2009 MacBook Pro and used the OWC Data Doubler. You don't need to provide a SATA cable, rather the frame (OptiBay or Data Doubler) used to mount the hard drive in place of the optical drive will use the SATA cable formerly employed by the optical drive.

As for tools, I believe you will only need a Phillips screwdriver (size #00 or #0, I can't recall). Unless they are required for the OptiBay, I don't think you'll need any Torx drivers. The installation instructions for the Data Doubler can be found here and cover the tools required for all MBP models. 

One tip for doing this procedure - use a long roll of tape stuck to your desk to keep track of the screws you remove at every step. If you're especially concerned about losing track of where things go, you can put the tape on a piece of paper and write down the location of each screw next to where you've taped it.

All the standard warnings about opening your computer apply here. In particular, be sure you have at least one backup of all your data.