This might work, but last I checked, current implementations of Intel's RST limits you to 32GB-64GB of cache space. Plus you won't see SSD speeds unless you run the program or game in question several times.2.
If this is RAID, I would never mix and match SSDs/HDDs especially from different manufacturers. Too many variables.3.
You could achieve a better effect by having a USB 3.0 2.5" enclosure for it... but yeah, this one seems kind of pointless.4.
If you just want to play around in the OS, a virtual machine would better suffice.
I'd use it as a secondary storage for games. If you don't want to reinstall games via Steam, create a symbolic link
that resides in the SteamApps folder that points to where the game will reside. Steam does have the option to install games elsewhere, but only on installation.