unfortunatly, we can't always trust the people at the fish stores. They aren't always dedicated fish keepers. Some places just rotate their staff through all the departments. So you could have been dealing with someone who normally works in birds. The best thing to do before buying any fish is to go on the internet and check to see what conditions they like... some fish can't live in the same type of water as others. They need different temperatures, different pH levels... there are many things. Not to mention that you need to know the size they will be when they are a full grown adult. It may be 1 inch now, but in a year it could be a foot long!
Don't worry about it. Unfortunately what you have now won't last long in your tank. If they don't kill eachother, they will quickly outgrow the tank and become unhealthy.
I would first look on the net to find out which of your fish will stay the smallest, and which are compatable with eachother. Take back as many to the store as you can, and only keep 3 or 4 hardy fish in there for a good 2 or 3 weeks to make sure you have a good amount of heathly bacteria that your tank needs to make sure your fish don't get sick. Make sure you do regular water changes and vaccuum your gravel to get the bad chemicals out of the water (ammonia, nitrates and nitrates). This isnt the best way to get your tank fish ready, but it will work... I always cycle my tank with fish.
After the 3 weeks, slowly add more fish in. Only 2 or 3 at a time, or a small school of fish that need to be in groups. Wait a few weeks, doing wather changes and gravel vacs, then add more.
A very general rule for fish stocking is 1inch of fully grown fish for 1 gallon of water. This doesn't always hold true. You can't have 1 fish that will get to be 30 inches. But you have to look at how big it could get... not how big it is now.
Hope that helps. Feel free to ask me any other questions.
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