9.8 mg/l calcium converts to 1.4 dH and 24.5 ppm. Some fish profiles use dH while others use ppm.
The best site for accurate information is Seriously Fish, which gives the hardness range of guppies as 143 to 536 ppm. At 24 ppm, your hardness is much too low for guppies.
pH is less important than hardness; fish can cope with pH outside their normal range better than a hardness outside their normal range, provided it is not too extreme.
You really need to get your own test kit, preferably one with liquid reagents not strip testers. This is something all fish keepers should have. Testing the water is the first thing to do if fish don't look well. For example, you mentioned guppies getting fin rot after adding more fish - was that due to a disease brought in or did adding new fish cause ammonia to rise, which would have caused problems for fish already badly stressed by being in water that is too soft? Testing for ammonia and nitrite would have answered that question in a few minutes. Unfortunately we can't rely on a shop saying your water is fine because so many of them have no idea what water should be.