I didn't really understand your question 2 but question 1: G E C is also C/G. Those slash chords, in which the the note following the slash is a chord member are chord inversions - and the only thing that determines which inversion it is, is the bass note. The distribution of the upper notes doesn't matter.
Yeah, I meant inversions...
Actually, I hadn't even noticed that you wrote 'intervals' instead of 'inversion'. Anyway it was your other question, I wasn't sure about. Are you talking about playing with a pick? I almost always use fingers so I just add on any note I want above any chord (provided I can reach it). There's a lot more freedom with (picking hand) fingers to do that as the melody note doesn't have to be on an adjacent string to the highest chord note.
By the way I just realised that last post was my 50th. I wonder if I get a prize :D
Use hybrid picking or mute the strings in between.
