chords-scales-harmony

Hi there,
I'm not quite new to ukulele playing (1yr) but now I try to understand the theory behind uke playing (which starts to seem interesting as well). I'd like to figure out what's the situation with chords.
The question is not the smartest one, so don't hesitate to tell me I'm getting whole the thing wrong
So the question is: are all the different chords also in some kind of harmonic scales as are notes? I mean if I pick, i should pick only in a scale (so it is harmonic), and when I play chords by strumming, is there also any "rule" how to combine the chords so the result was harmonic? For example for some genres or that you just can't combine some chords because it sounds bad...

I'd be thankful so I can move on with my playing which is just for fun (but for which the theory opens new dimensions as well).
You don't have to explain alle the stuff (if it is worth explaining at all), I'll be glad for links and sites as well.

Thanks

Welcome to UU!! Howlin Hobbit has a very good primer on music theory that he calls Cheater Theory. You can find it here. Also check out the stuff that NukeDOC put together in this thread.