I prefer metal on any straight beads, plastic on radius.
Best if you use a "hot mud" (durobond setting type) for the first and second coats, this will give a strong corner w/o drying cracks, and you can do both coats the same day.
Sure you can join 2 beads. I use a 3x5 piece of aluminum flashing folded @ 90 degree angle under the seamto keep them lined up. Make sure you don't have any burrs on the ends, if so, a little filing will do well.