My two cents:
A freshly cycled tank has a bio filter capable of handling whatever bioload you put in there to cycle it. If you add more bioload, it takes some time for the bacteria to multiply enough to handle the new increased bioload. This happens by ammonia building up and causing the bacteria to increase until a new equilibrium is established. If you add many fish, the ammonia will spike faster and higher than if you only add 1 fish, until the bacteria can catch up. In a small freshly cycled tank, there is very little bioload, and therefore very little bacteria. Adding 4 or 5 fish is a mistake, especially in a tiny tank like a 10 gallon. I am not saying it can't be done, but aside from the aggression issues previously stated, the bioload can easily get out of hand. If you do decide to add all at once, make sure that you monitor ammonia carefully and frequently, and have some type of ammonia removal on-hand, or buy some bacteria to help seed the population and still keep some ammonia removal on hand..