well the Neons and the cardinal are schooling fish, and should probably be in a group of at the very least 5 to be hapy, the Neons and the cardinal may school b/c they look so alike, but you still need two more even with that in consideration. Also, Is your tank cycled? I saw on your aquarium infromation that you didn't know what your readings were. To find out more about the nitrogen cycle, go here:
http://www.fishlore.com/NitrogenCycle.htm
As for what I am putting in my tank, I am thinking of a betta and 3-4 cories or 5-6
male fancy guppies. For your question about having too many fish, the general rule is 1" of fish per gallon of water, so for your 10 gallon tank thats 10" of fish. you should always go with the full average adult size for calculating the inches of fish in your tank. Like, for instance, If I got 2 mollies, a guppy and 2 cories, the cories, getting to an average of 2.5", would add up to 5", the Guppy, would add another 2", and the mollies would add another 6" as they get up to 3" each. this would add up to 13". this lineup would be over-stocked, but not so much that you couldn't handle it with doing more frequent water changes and keeping a close eye on the ammonia and nitrites.
Unfortunatly, I have calculated How your tank is stocked, and you have a potential of 17" of fish in your tank. this would be near impossible to keep healthy. I would reccommend getting rid of at least the tetras if you want a healthy tank, as without the tetras you would be down at 12" of fish.
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