Pam, I have used Perle cotton and DMC on different projects. The size Perle Cotton you use really depends more on your quilt than anything. If I was tying a denim quilt I would use a thicker heavier thread. When using DMC floss I have used all 6 strands. I start with my thread on top of the quilt, take a tiny tacking stitch, leaving my thread tail on the top of the quilt, then a second little stitch in the same place. I then tie my thread tails in a knot and cut off, leaving about a 3/8" tail in place.
Caution---some DMC floss will fade. I have had trouble with red more than any other color. I always soak my floss in cool water with a tiny bit of detergent in it. If it fades I set it with vinegar, or try another color!
Hope this helps.