S'mon said:
I can't think of anything in GT that would be a good fit with C&C.
Oh I wouldn't go nearly that far, but from what I've seen you and I hold opposite views on quite a few things (everything?).
I can see the value of adding some stuff from Grim Tales to C&C, it's just that everything that would be added is rules. The rules themselves are fine, and certainly do what they set out to do.
I just don't think that anyone really looking at C&C because they want a rules light system should be starting off promptly adding rules right back in. That's not making a rules light (or lighter) game, that's changing the rules focus of the game, which is a bit of a different proposition.
If the OP is actually looking for a d20-based system that he can "rebuild" to better suit his sensibilities, then C&C isn't a bad way to go. It certainly won't be liked by the majority of people on either side of the C&C vs D&D group, both of whom (right or wrong) would say something like, "Why don't you play True 20" or some other system that seems to aim at being d20 but lighter.
Part of the problem is that a lot of stuff is "exception based" when it comes to d20. This feat or that means you don't suffer the penalty that everyone else does when doing something, or this or that feat gives you an explicit permission to do something, which in turn implies that anyone without the special permission (feat) can't. Adding in things like Talent trees and whatnot starts to head in the direction of re-complicating things that C&C deliberately left out.
It's been my observation that some people seem to "like the simplicity that C&C has" and then want to plug in their own reworks of this or that d20 rule and promptly recomplicate it, only in a direction that suits their own particular quirks. There's nothing wrong with that (I'm a strong proponent of houserules), but if it's the sort of thing they're planning on doing, they should be honest with themselves about it. Otherwise you wind up with a mess of rules that individually are fine, but conflict and aren't clear when put together.
For example:
I like the simplicity of C&C. I did 2 major rebuilds of C&C systems. The first was the magic, and the second was class building.
For magic, the goal was to allow me to take any kind of magic whatever, and be able to plug it into C&C and have it work. I used the Elements of Magic (Revised) for that. When I was done, the C&C spells basically looked the same, and the magic system wasn't any different. But I now had a way to be able to introduce different magic systems, and have a reasonable confidence that the spells/powers would be on roughly the same level.
In the case of the class building, my goal was to be able to take a class out of whatever D&D/d20 book I felt like, and be able to bring it across to C&C and again have at least a rough confidence that the classes would be equivalent. I used Buy the Numbers for this.
In both cases I used some rules and guidelines that are _much_ more complicated than C&C is and applied them. They weren't rules that the players would ever really see or be aware of, and they weren't anything that I'd have to mess around with a lot as the game played. But they were rules, they were present, and they influenced how things looked from my perspective and how certain aspects ran.
It's possible to take rules and complications from other sources, and feed them back into C&C. You can do it in such a fashion as to overtly complicate things, by reintroducing a feat system (even if it's limited) or reintroducing some sort of skill system (another popular tweak by different C&Cers). It is possible to maintain the simplicity of C&C, while having more complicated systems that are buried in the background that the players and (usually) the GM don't have to mess with.
The OP should be clear as to what direction he wants to go, if he's looking at mixing Grim Tales with C&C. It's going to influcence how he translates/rebuilds the rules, as well as whether the game still "feels" as if it's as "simple" or "uncomplicated" as C&C is by default, or simply a lighter version of d20 that started from a C&C base.