Not too worried about that, each nation and time period has special rules. Just off the top of my head, Pacific has caves and banzai charges and jamming machine guns; Blitzkrieg has hit-and-run movement and cavalry; the modern stuff has helicopters and multiple ammo types for vehicles. Lots of things to add.
So from this we could assume that yes, different rule sets or alterations to rules come with new purchased games. However as far as modding those games goes, "have at it".
Modding the AI is something I would like too - at the moment I don't understand what's going on with the AI at times when it seems to just "pass" and lose the game - other times it goes for the jugular. AI routines are usually hard coded into a game and not so easy to modify.
The editor did seem a mystery to me for a couple hours but I'm familiar with the basics of it now thanks to reading Tom's guide and also jonj01's (thank you!) youtube demonstrations. The editor isn't worse than many I've encountered in other games. As pointed out earlier modders and scenario designers tend to be devoted enough to surmount difficulties like obscure editing tools. Even I have decided it's possible to experiment with making scenarios and I'm dusting off my rusty photoshop skills to try my hand at making a map. Although the superlative maps & counters already available here could be hard to live up to.