I know what you mean about putting fabrics together that are not always right with each other, which is why I am drawn to fat quarter bundles.
If I want a primitive or country look I tend to be drawn to Kansas Troubles Quilters fabrics. The fabrics are just amazing and there is no way to not put them together that isn't just perfect. Most of her fabric lines are red, gold, blue, black, eggplant, green, and dark tans. Sometimes she throws in rust (my favorite color). Most of the time the fabric line has 40 prints, so a fat quarter bundle is perfect.
If I want a civil war sort of look I tend to be drawn to Jo Morton and Judie Rothermel (spelling?) fabric lines. Buying any of their fabrics (doesn't matter if it comes from the same fabric lines) will always work together.
So, a rule of thumb for me is to pick a designer you like and buy their fabrics and most often they always "play well" with each other. FYI, I think Jo and Judie's fabrics play well together too.
Suzanne