
Most danios get to 2", different types of tetras can get that size or bigger. Your bioload would be overstocked. They wouldn't have enough room to swim freely in a five gallon. The overcrowding would cause undue stress, which would lead to a weakened immune system and your fish would not be able to fight off disease. And finally, most tropicals perfer to be kept in groups, which you just don't have room for in a five.
You could possibly squeeze five male guppies in a five gallon, but four would be better. And make sure you get all males (you know how to identify them yourself right?)
Another option might be,,, I'm assuming your getting a 5 gallon kit, you may want to look at the 7 gallon kit, I don't think the prices are that different. If you did some shopping around, you might even be able to find a ten gallon kit even cheaper! Look around. A five gallon makes it harder to keep your water parameters in line.