Re: fishing a dropper
The fly you choose to use on top of a dropper rig is dependent on a few factors. When deciding on the pattern to use to suspend your dropper nymph consider the weight of the dropper and kind of water you are fishing then choose a pattern to match that. A small #16 prince dropper fished on relatively calm water may only need a well hackled #12 dry fly like a Wulff, humpy, or even standard Adams. I like to use a stimulator for the top fly, it floats high, is visible and is effective in small to large sizes.
When fishing a dry/dropper rig you are always fishing one of the flies as you primary and the other as a secondary fly to add to your odds. You will either be presenting the dry where you think a fish will rise and take it or cast where you can get the nymph to drift where you want it. Rarely will you be able to present both flies in a prime spot simultaneously. With that in mind, you will have to decide what is most important at the moment.
If you are primarily fishing the nymph but want a dry for the indicator and random chance at a rise, match the size and buoyancy of the top fly to the nymph. If you are fishing more with the dry but also want the extra chance of a hook-up on the nymph then choose the size of nymph that won't pull your dry under.
As far as using a double dry rig in order to locate a small fly, just use something that is visible but makes sense on the water! If you are primarily fishing a #18 dry because that is what the fish want, then you don't want to tie it 8 inches behind a big foam hopper or chernobyl ant which will scare away trout feeding on a hatch of small insects. Use a something that you can see but is as unobtrusive as possible, maybe that is a #14 parachute with a nice white post you can see, maybe you can get away with a visible #16 caddis. The point is to use as little as possible as your indicator fly in this situation.