[Buggy]

The "one inch per gallon" rule can loosely apply to the smaller bodied fish such as some of the smaller tetras. The problem with this is, they need schools of 6+ so you still go over your limit. Deeper bodied fish don't count in that rule. Also you have to take into account the full adult size of the fish. Most fish you buy at the pet stores are juveniles and will get bigger in time. What you are really looking at is the amount of waste the fish put on the bio of the tank rather then the room they have to move. Some fish are just naturally messier then others and will overload a small tank like that.

You can do a fishless cycle by wrapping a piece of c0cktail shrimp in a nylon stocking and dropping it in the tank. As it decomposes it will release ammonia and start the cycling process. Keep checking the ammonia and nitrite levels every few days and when they both read 0 and you have a nitrate reading do a 15% water change and you will be good to go.