It's not clear to me from the video which notes of a triad to select to do chops to support the tune. So for example, with Handsome Molly in key of D, when the group is playing the D chord, on the fiddle, do I have a choice of supporting that with cops on open a and d strings? can I choose to do f# on the d string and open a? or f# on the e string and open a? or a on the g string and F# on the d? are all of those acceptable?
Then when the song switches to the A chord, can I support thay with any notes from an A, E, C# triad?
Since the fiddle is cntributing rhyhtm back up is the role of the fiddle in a jam only to play chops on lower strings? Or are fiddles allowed to play chops on higher strings?
Also, how much should I mute the chord I am chopping on? Should the notes come through or shoukd it be more like Derrel Anger's chopping where it's pure rhythm?
Yes, any combination of notes that make up the current chord will work as a chop. Sticking with the simplest combination of notes (say, the open D and A strings, or the open D string plus G string first finger) is just fine and honestly that's almost always what I do… but if you know your way around the scales enough to make up your own double stops to chop over, sure, go for it!
Since the fiddle is contributing rhythm back up is the role of the fiddle in a jam only to play chops on lower strings? Or are fiddles allowed to play chops on higher strings?
I usually prefer lower strings because of the sound, but it's a minor difference so follow your ear.
Also, how much should I mute the chord I am chopping on? Should the notes come through or should it be more like Darol Anger's chopping where it's pure rhythm?
Uh oh, there's that word “should”. :-) This is totally a matter of taste and preference. Personally, I do a mix of both, erring on the side of a more percussive, less “note-y” chop in bluegrass contexts at least. In fact, half the time I just lay all my fingers over the strings to mute them completely for a purely percussive chop!