i used to use a silver arrowana as a dither fish for my datnoids and it worked well (not shy and great color). too bad the arrowana got too big so i sold him and now my dats arent as stable as they use to be.
One exception to the caveat of bottomfish not making good dithers is a group of clown loaches, they are active and pushy enough to stir up motion in fish that stay hidden.