over the last two evenings,I have played at 2 sessions, the thursday night session was a delight not one accompanist other than a very good bodhran player, there was no speeding up during tunes, all the playing   was steady the standard was very high.
last night by contrast I along with 2 other competent fiddlers was trying to have to hold the session together not only against guitarists who were occasionally clueless, but also speeders up, I consider myself no more than a competent / good player , but it was hard work with people [a few of whom] were not listening carefully to rhythm and melody.
mixolydian tunes should be accompanied by major chords or power dyads , dorian tunes minor chords or power dyads, sometimes of course tunes change from one mode to another during the tune the best thing is not to join in unless you know the tune or have heard the tune or if you do join in,do it very quietly.