I really have to wonder how shops like that stay in business. Customer service is one way to have a leg up on the shop down the street, now you're going to that shop down the street, P@H. The barbs need to be in a larger group, or they will get nippy with the guppys, in a larger group this aggressiveness bit is spread among the larger group so it isn't an issue.
An occasional dead fish in a shop is fairly normal, considering they are more of a holding facility for sales than a long term fishkeeping setup. Most get fish shipped in at least once weekly, depending on the health of these fish as well as the care given before arriving there may be no losses, or there may be problems.
Look for the API master test kit, this is a liquid test kit, the strips are cheaper but unreliable. The API kit is pretty much a standard piece of fishkeeping equipment.
Snails are a common thing with plants, as the eggs hitch a ride with the plants even if there are no snails apparent. I don't do plants, all but 3 of my tanks have no substrate. I do have quite a few snails in my bristlenose breeding tank, about once a week I collect a bunch & toss them in a loach tank. I'll ask around about snail prevention with new plants, as I recall it's a dip before adding to the tank.
Most places will bag species separate, with larger or more aggressive fish they will get bagged as individuals, guppys & barbs in the same bag is a bad idea. Look around at other shops, some are pretty lame, some are so-so, some are really good. When you find a good one stick with them, they are the ones that deserve your hard earned money.
The fact that you're doing research now already means you're probably doing a lot more than most people who just listen to the crap advice given to them by the sales people! Good luck!
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