@Wayfarer try using Gutermann polyester thread. It’s a much better quality thread. Moon thread is thicker. As for a 6mm needle, I wouldn’t try that if you’re moving needle positions or you will probably hit the presser foot!
There are different types of twin needles just as there are single needles. Universal, embroidery, stretch, jersey, jeans. I’d be inclined to try a jeans needle on the leather as it will go through easier than a universal.
If this is just for decoration why not just do 2 separate rows of straight stitching?
I’m personally not a fan of twin needle work but I do have a coverstitch machine so don’t need it