when i finger-pick jigs on the guitar I play TITTIT, which is the same as what most people do with flat picks on guitar or tenor banjo for 6/8. Leading with your finger on a 5-string is never going to get the beat you want, I should imagine: you'll get the notes, but it won't come out so jiggy. Lead with your thumb, it does the work for you. Mexican 6/8 is another thing, by the way: I'm only talking about the British/Irish traditions.