Aaaah, I'm so sorry to hear this, dear. :( I wonder if he was already on his way out, and that's why he was being targeted in the first place? Poor little thing. . .
Mmmm, as for Calcium. . . I do NOT have an answer for you. From my understanding, the only way to really do it right is to increase the hardness (calcium content) of your water! I can't do this, because of the other creatures I keep :/ For the time being, they're all fine. The Olive has some chips on her shell - she came to me that way, I've seen no signs of trouble since I got them - but it's only been a few months since I started my first tank (I read a lot ;)) I have plans to make a hard/salinated tank for my Mollies (purchased before I knew anything), and I'll probably be putting my Nerite snails in there for their own health and happiness. People try to supplement the diet of the snails with calcium, but I have no idea if this really works. I also don't know if the creature inside of the shell is caused harm from living in soft-water conditions - but sadly, I think that it almost has to have a negative effect on them - it isn't the kind of water they were made for. Which sucks, because (as you might have guessed) I REALLY like snails! I think they're super cool! I also have a small army of Malaysian Trumpet Snails (I think there are pics of these guys in my aquariums - the 29) living in my tanks - those I will never be able to rehome, so I hope they continue to do okay. . .
As for personalities - they do - but all of mine are different, so I don't know what the 'norm' is! I had Apples and Nerites in the same tank, and while the Apple was about, the Nerite was very much a ninja snail. I NEVER saw him move. I'd look at the tank, and he'd just be sitting wherever. . . I'd glance away and he'd be on the other side of the tank - but I never saw him move! For months. . . after the Apple died, he started moving around a lot more, like constantly. . . I don't know if this is because there was more algae in my tank after the Apple stopped eating it or if it was a territory thing, but now he trucks all over the tank. They tend to be invisible, as a general rule. I don't think they hide, exactly. . . they just blend in really well - while the Apple's stand out. My Olive Nerite has always been VERY active. *shrug* Apples have more personality, just because you can see their little faces peeking out. They're hilarious when they climb onto (and fall off of) grasses and such - the Nerites. . . are more serious than that. I've never seen one do anything silly. My girls and I really liked watching our Apples, and we really like trying to find our Nerites! I love them - and they seem to stay alive for me, so I'm good with that.
Other odd things that I can think of. . .
They don't have a foot, and their shells come down much farther than the Apple's do - you can only just see them underneath when they're moving - more of just a magic shell ;) You want to make sure they don't fall upside-down, cuz' they may not be able to flip over again.
They're tidal snails, so can live out of the water for short periods of time, and can easily escape a tank, if you aren't prepared for it. The theory is that they won't try to leave if they're happy - mine don't, and they easily could if they wanted to. A couple of mine stayed juuuuuust above the waterline when I first introduced them to the tank for a few days, but once they got used to things, they were okay with it, and I haven't seen them out of the water since.
Sometimes when they're introduced to a tank they go into a state of dormancy for a day or three before they get used to things and decide it's going to be okay. . .
If you can get them at PetCo, make sure - these snails can live in fresh or brackish water - and anything in-between. They only breed in brackish, though - I'm not sure if PetCo keeps them in their tropical or saltwater section - or if it matters. I have no idea how to tell gender - aside from the fact that the girlies lay eggs ;) Technically, both Stripe and Olive Oil are females. I've read that you can tell gender, as the male's 'junk' is folded up neatly by his antanae - but I guess if I have any boys, they've never taken it out when I was looking!