(By the bye, this is a diary, so I voted it down as a log submission or supposedly multi-page article. But I'm glad you are here. Do these as diaries for even faster responses.)

I would think that fancier guppies would be more appealing and would therefore get more attention and seem heartier, in that we give them more care. However fancy guppies are often touchier. Their tolerance for dirty water is less than that of feeder/ wild type guppies. Their susceptibility to a wide range of subsequent diseases (not the least of which include bacterial infections such as tailrot) is much greater.

Lomelindi is right on in the assertion that feeders of any kind (not just guppies) are swimming museums of fish diseases. But both fancy and common guppies should be quarantined and treated with an anti-parasite treatment. I would recommend going with something designed for internal parasites. Check with your pet shop. If they don't know if they have such an item (they very likely do), leave and find a shop, which at least knows that basic a thing.

A strain of fish, often considered to be guppies, which is as tough as the feeder guppies, is (actually are now) the "Endler's livebearer(s)". While they will cross with guppies (a lot of fish will cross in aquariums, where males can force themselves upon females), a recent field study convinced three scientists to stick their collective necks out and scientifically describe these quite variable fish as a new species - Poecilia wingei. (There is a log on that currently on the Guppylog front page.)

They put up with crowding and, if that field study  is reliable, are more tolerant of one another and the males are less rough on the females. However, they will not drop fry until they are fairly warm. My fishroom is often fairly cool by aquaristic standards and they didn't drop until the warm months rolled around. I don't have the precise temperature, but they will begin dropping about in the middle '70s F / 24+ C. Once they get going, the only way to slow them down (other than threatening the wife and kids) would be to gradually cool them down to the lower 70s, something that also slows down growth and maturation.

It is possible to neglect them to the point where they will catch illnesses. The first outbreak of Ich I had seem (here) in 30 years was in a crowded, very dirty Endler's tank. I had really taken their hardiness for granted and the temperature drop provided by an open window in the fall, effectively compromised their immune systems enough that the Ich spread white spots all over them, took a lot of the residents, and only left a well populated aquarium.