There's a popular tying thread called Monocord, approx. 3/0 and usually unwaxed. It's used for tying larger flies. That's excellent for spinning-bobbin wraps. Anything of similar size is also suitable. Since the thread is protected by a top coat, its size is not critical. I'd trust 6/0 for most freshwater lines.
The spinning bobbin method puts over 100 wraps in a one-inch splice. That's a lot of gription. I've found that a 1/2" splice is stronger than 20 lb. test leader. I coat splices with about six coats of clear nail hardener, found in the cosmetics section. Each coat dries quickly, and is flexible.