I'm considering buying the whole lot but I'm also wondering if its really worth it, how do they layer amongst each other
The (remastered) Cathcart timetable and the 158 timetable for Edinburgh-Glasgow basically use all the Scotrail layers. Fife Circle doesn’t for now, but will do when the mythical 158 timetable shows up.
With the 158 timetable, owning the Fife Circle route gives you drivable 170 services between Glasgow and Croy, owning Great Western Express gives you drivable HST services also between Glasgow and Croy. If you have the 380 you get some class 380 services, and of course you get 385 (the train that comes with the route originally) and 158 services. The only Scotrail train you don't get is the 314 from Catchcart, although there is 1 service for that on the base timetable. You'll also get drivable LNER Azuma services from Edinburgh to Polmont if you have East Coast Mainline.