Well, it depends on how large the tank is. If the tank is, say, 75 or 90 gallons, I don't think it would be dangerous to add to it 6 small schooling fish (like neon tetras, for example). But if the tank is only 10 - 20 gallons, adding 6 even small fish at once may be dangerous. How large is your tank? If it's anywhere between 10 - 30 gallons, I'd put only half of the school at first, then - maybe a week later - put the other half of the school in. If the tank is 10 gallons, I wouldn't put in there more than 2 - 3 small fish at first.
If you add too many fish, even though the tank is cycled,
ammonia and/or
nitrite can suddenly go up. It is for this reason why it's important to stock only few fish at a time. It is also very important to
acclimatize properly any new fish that you get. Please remember to acclimatize. Here is some information on how to do this:
http://www.fishlore.com/acclimating-tropicalfish.htm.