What would the fastest/most efficient allocation of files and OS (Windows 10) be for editing in Premiere CC on a laptop with a TB 7200 RPM Sata and 128 GB SSD? I'm wondering whether the OS should be on the SATA or SSD and where to put project files/cache, as well as which drive to export to. Any advice is much appreciated.

The rest of the specs of this machine are:
Intel® Core™ i7-8750H
NVIDIA® GeForce® GTX 1050
16 GB RAM

You would be better off using the SSD for the OS. it depends on how many programs you're going to install, My SSD holds the OS and all my programs and the used space is currently at 468 Gb.
Theres a bunch of other things to consider too like the life of SSD v HDD, prices etc.