Then there is the question, why do you want to back up these fiddlers? In general, they have little use for the guitar, unless you are playing for dancing(and they will "allow" you to play as long as it is understood who is in charge)--you are never going to get to do anything except play chords behind them, and if you try, God help you!

If you want to play old timey stuff, you'll do better working with a different instrument where you can stand on a more equal footing-- best to find a mandolinist, who will know the same tunes, but won't be able to get by without a strong rhythm player,and will probably want to sing--also, you can play solo bass runs behind those plinky mando chords and feel somewhat important(of course, the mando will play most of the melody and leads)--

A banjo player isn't bad to work with, either--they can keep the beat and solo at the same time, and don't really need you, but they tend to be a lot more sociable--

A steel guitar player would be good, too, though it will be hard to find one in the Islands who doesn't want to just play Hawaiian style(not a bad thing)--

Nothing against them as musicians, but for fiddlers, music is about the fiddle, and guitars are more or less the fifth wheel--