I'd probably knot between each pearl, even if very small. Hopefully the hole size would also be small, so the thread can be in proportion. The smallest I've knotted between each pearl so far has been an eyeglass holder for myself, 3.5-4mm, 25 inches long.
Certainly power pro or fireline are good options for strength. Many beaders seem to prefer fireline over powerpro. I'm going to pick some up at our newly opened Cabelas and try it out.
Of all the reasons to knot between pearls, flexibility and drape seems so important, yet it is seldom, if ever, mentioned. With smaller pearls and no knots, one still has some flexibility, which is lost as pearls get larger.