Failed to load mods: FortressRuins/settings.lua:1: attempt to index field 'AbandonedRuins-set' (a nil value)
stack traceback:
FortressRuins/settings.lua:1: in main chunk

Mods to be disabled:
• FortressRuins (1.0.8)

The settings key has changed. I can recommend to import __AbandonedRuins_updated_fork__/lua/constants.lua and then use CURRENT_RUIN_SET_KEY instead of static "key-foo-set".

Then I would also recommend to use add_ruin_sets and not the already deprecated add_ruin_set. I deprecated it because it only allows 3 different sizes of ruins, "small", "medium" and "large". With the other function, you only need to handle your ruin-set's name and the whole table base_ruins.

Im going to take a look at the new changes you made to the main ruins mod as It looks like you have enabled per planet ruins which I think is great! so its likely I will refactor my ruins mods to take advantage of this new feature

Yes, you can allow each ruin individually to spawn on only a few planets or mark your whole ruin-set as exclusive to a planet. You can find its documentation here:
https://github.com/Quix0r/AbandonedRuins-base/blob/master/docs/ruin_sets.md

EDIT: I'm still working on to improve the documentation. So if you come across any obstacles or even know how to improve, I'm happy to receive a PR (pull-request) from you at github. :-)