Having CT'd for a bit, I've noticed that kits by the same designer can mix & match easier with each other than grabbing kits from another designer. Similar styles can also match easier. Also - if a color doesn't match, then I'll just alter it to match (if I really must have it on the page). It can be harder (and I imagine tagging would help me tons - haven't done it yet) but it can work if you keep at it
... Most of the time, I don't go into it specifically to mix two or more kits. I'll just get going and realize I want some ricrac or maybe a different ribbon, and then I'll go on the hunt for one
Hope that helps