Not to Discourage you or anything, but with the plants you describe you will need more light. With that kind of light I'd be doing anubians and moss and ferns.. stuff like that.. Anyways Back on subject. I would think that well water would be harder than that, but this is good. Unfortunately I am not too familiar with the calcium product that you are adding, but with the tap water parameters you describe I see why you might want to raise the General hardness only.
Honestly I think that the KH of the tap is good. I'm wondering if the product you are adding is causing the KH to go up to (along with the GH). If I were you I would do a little experiment. Take that calcium and dose a bucket of your tap with it, wait a while and take some measurements. I have a hunch that this may be causing the KH to go up, but doing this will check. Another thing that can cause this KH rise is any type of rocks with carbonate in them. If they are not totally inert they can cause problems with the KH going up. This is something to check out too.
Regardless I personally wouldn't recommend dosing just calcium to raise the GH because your plants and fish may be missing out on other minerals that are in trace amounts that also are part of the GH puzzle. Like Magnesium for example. You may want to dose that too. I personally use Seachem product called Equilibrium to raise the GH because it has everything the plants and fishs (snails too!!) need from a higher GH without having to mix all the individual elements yourself. I have had excellent results and I don't have to worry about getting the ratios of cal:mag right because it's done already. I also will not touch the KH.
Once you get the source of you KH identified you should be able to remedy the problem with 40% weekly water changes. Good luck!
O yeah since you are concerned of the snail health- (after a while they will reproduce so fast you'll want them to die LOL
) one thing that you can add is a little bit of Iodine. This helps their shells and is beneficial to all verts, but only use a little bit so it doesn't harm the fish. I think it's 1 teaspoon/50 gallons each month.[/b]