Hello,
Ducks are great! I think any sort of duck will do fine in your conditions, as long as they are provided with plenty of clean water and SHADE. They don't like heat particularly, but they'll do fine if they can bathe and sit in the shade. The winter won't bother them a bit--they are well insulated.
A plastic swimming pool is perfect, because you can simply dump & refill every few days to keep the water fresh. If you're in high desert, however, I suspect you are on water restrictions, so you'll want to find a way to recycle the water (it makes AWESOME fertilizer--just pour directly on needy plants). And choose a small pool, since you only have a few ducks--I have had as many as ten sharing a small molded plastic kid's pool, the sort that are shaped like a turtle or frog from
Walmart and such (except I salvaged mine from the roadside, lol).
Most ducks are pretty hardy, but yes, you'll need at least two. A single duck will bond too strongly with you and/or the chickens, and never be truly happy. The chickens are unlikely to accept the duck as a flock mate (although they will probably tolerate a pair just fine), and a single duck will be lonely and miserable.
For color, consider an ancona or magpie--very cute, and good layers too. Or try an Indian Runner--very popular, widely available, loads of personality, and a conversation starter (their upright stance makes them look like little people, or like wine bottles--very funny). Runners come in a variety of interesting colors (white, mallard, black, fawn & white, blue, silver, chocolate...) that mix & match nicely.
Welsh Harlequins come in only two main color varieties, but they are beautiful. All three breeds mentioned are good layers.
Have fun. Definitely do some research like you suggested. Also, find yourself a copy of Storey's Guide to Raising Ducks by Dave Holderread. It is the bible of duck keeping and will answer nearly any question you might have.
