Why would that only effect Guppies though and not the other fish I have kept? Are Guppies more sensitive due to genetic weakness?
Not necessarily, but since you said you bought the others at your regular lfs and the guppies from a different place- not your normal, it leads me too believe your usual lfs is closer to you and potentially share the same water supply. The other possibility is that your current tank inhabitants have adjusted to less than optimal conditions over time. I wouldn't rule out bad stock 100% but at least knowing the potential water toxicity is usually the first step in figuring out why you have such a different experience with them than the lfs they came from.
I just recently had a terrible experience with my tank, and I knew it was going to have a problem, but it was kind of an experiment, when I started stocking my tank. I know, from being in the aquarium industry (breeding/ plant cultivation, set up/ maintenance), that "pop and drop" or "drop and plop" was going to cycle me. I told my hubby and my child it was going to happen and that I was being a complete idiot. I didn't acclimate my first inhabitants. The tank they were going into was perfectly cycles and had been up and running for the better part of 10 years. 0 ammo, 0 nitrite, 25 for nitrates, moderately planted. I dropped the bags of 8 otos and 8 cories into the tanks, floated them to adjust water temp, cut open the bags and let them out. By the next morning 2 cories were dead. Tested my water and ammonia was through the roof, nitrites were starting to show... I had thrown myself into a cycle. Surprisingly every oto survived. Unfortunately I lost 4 cories. I tested the water from the lfs I used and their parameters were toxic- 5 for ammo, trace nitrite and trace nitrate- basically in perpetual cycle. When I went there to replace the cories I made sure to acclimate them in a bucket, net them and release them in my tank- no deaths. After that I did the same acclimation process with my rummy nose tetra and cardinal tetra- lost 2 rummynose and 1 cardinal, they were basically dead in the bags when I got home and I expected it. I hope this story can shed some light on why acclimation is important. I wish you the best!!