I'm experimenting with advanced mapping and I want to make a hybrid rules set for one of my maps that has a mix of the original Red Alert rules and the openra rules
It would be difficult to compile such a list as the RA mod is quite different from the vanilla Red Alert game. However you can have a look at https://github.com/OpenRA/raclassic - it is a mod for OpenRA that strives to recreate the classic RA gameplay and balance.
Looking at raclassic is indeed the best idea here.