First - You will bring in fungus gnats for sure with your potted poinsettias and amaryllis. Though the large producers make an attempt to control them, it is inevitable that you will get one or two and they will multiply! Your suggestions of running your plants on the dry side is a good one- especially with poinsettias and amaryllis - those are the conditions they prefer in our low light homes.
A first line of defense would be to replace the top inch or so of the potting medium immediately. New potting soil is fine, there is no need to use sand. Sand would only keep the top layer dryer. The adults will lay eggs in it if it is moist. The fungus gnat larva reside in the top layer where they can get oxygen. They don't go much deeper because conditions are too anaerobic. Also, fungus gnats feed mostly on decaying organic matter, with an occasional snack on roots. Especially dying roots from overwatering. Do we see a correlation here? Uh-huh!
Sticky traps will slow down the adult population only - but that helps. The best control is predatory nematodes applied to the soil surface. These little tiny bug-seeking-missiles-of-doom ferret out the fungus gnat larvae and re-create some thing from a Steven Spielberg movie. I leave the rest to your imagination. Scanmask is a tradename. We carry it at Johnson's in NW and it is available on line. The only downside is that it makes a large quantity, but it stores in the refrigerator - the nematodes are in a dry stasis, no need to fear for your lasagna - for 20 months.
As for whiteflies and scale - a wiping down of the leaves will help, but a magnifying lens to see what you are doing would improve your chances of getting rid of eggs, pupae and larvae. I suggest you use a neem or horticultural oil spray instead of insecticidal soap. The soaps are pretty useless on any phase other than larvae and adults (whitefly) and only larva (crawlers) of scale insects. The eggs and pupae will go unmolested. Insecticidal soap has been known to damage new plant growth, another reason why most IPM folks prefer the oils.
Thanks, Carol. Many of you local DC-area gardeners may know Carol (aka The Orchid Lady) from her frequent orchid repotting workshops and her talks she gives at many local area garden clubs. Which reminds me that I'm booking the 2009 Silver Spring Garden Club speaker schedule next week and I need to add Carol on to that roster.
Photo by Jack Dykinga, ARS.USDA.GOV. Pictured is a tiny pirate bug, Oris insidiosus, feeding on whitefly nymphs.