Don't have a general hardness test kit, but I checked my supplier's water report quality and putting thier highest detected ppm of calcium and magnesium in the calculator here,
https://www.lenntech.com/ro/water-hardness.htm
Gave me 42.5 ppm of hardness as CaCo3, or 2.35 degrees German Hardness.
This is stated as either slightly hard or moderately soft water, depending on which website I reference.
I have no experience with that converter, so will leave that aside. If the GH is accurately 42 ppm (= 2.3 dGH) then it is very soft water, not "moderately soft" or "slightly hard." Mind you, these are subjective terms that can mean different things depending who uses them, but the numbers are without question very soft water. Cories will have no issues with this aspect.
Since my PH tends to be fairly stable, I suspect an additive to raise PH to 8.5, especially since the lowest PH detected for the year was 6.8
I would confirm this with your water authority, as it does seem likely something is being added. I have that here, sodium ash is used which dissipates out and my pH is in the 5's or 6's depending upon the tank. Fine for soft water species. Which brings me to the guppies...no. They will not be happy in such soft water, and while they
may survive, they also may not, and they will not be in good health if they do survive.
Do you think it would be too much to add some decorative shells rated as aquarium safe to further stabilize the PH?
No. First off, you need to confirm the GH, and figure out the pH issue. Then, go with fish that are suited to the parameters. There is no point in making life more difficult for soft water fish just to have some harder water fish in the tank. No one (thinking the fish) will be happy, and this makes their lives much harder which should not be our intent.
On the temperature issue, I agree that non-permanent temperature increases are usually manageable by fish.
Corydoras panda is one of the cooler water species, coming from mountain streams in Peru. I have mine at 24.5 C year-round, sometimes during heat waves in summer a few degrees higher or lower (heaters are disconnected during warm months so temperature varies diurnally). This has been the case for 10 years so far with this species. They are in with a mix of cory species and I suspect have cross-spawned as well.
You mention pygmy cory...these are more delicate even than the larger species tend to be, and temperature is crucial. My 10g of
Corydoras pygmaeus runs at 24C (75 F) and this is as high as you should have it for these cories. And they must have sand.