As near as I can determine, guppies are very hardy once you get them going. Stress is the greatest killer of guppies - that and the "guppy plague" - which seems to be a combination of stress and bacteria.
I have lost 60% of all guppies given to me - they die within 24 hours of shipment. If they are in contact with any others, I have often lost the ones they were in contact with. This leads me to believe they also are carrying some infection that comes to the surface when stress lowers the immunity.
If I can get a guppy past the first week, (the first 24 hours!) it usually survives. Once again - quarantine was broken and had a horrible outcome.

As new fish arrive, I treat immediately with Melefix and isolation. They are kept warm, dark and as unstressed as possible after arriving. I have had a fair amount of success with this regiment, but having a new one tossed into an established tank was just the wrong thing to do. His heart was in the right place, but it was death to the guppies. Sigh.
I have a set of beautiful leopard tuxedo reds and a set of lyre tail sunrise that are thriving. But the moscow lines - very large, strong (supposedly) and solid red or blue guppies have been my downfall. I do so love those strong colors. I will get a pair to live and breed eventually. I just hate losing all the others before that time.