I tried a high-speed Izzet deck once. What I did was try to take advantage of Nivmagus Elemental. Have a bunch of 1-drop instants like Downsize, Blustersquall, Dynacharge, and so on. That way, with any luck, I'd have a 5/6 swinging on turn 2. Guttersnipe and Talrand, Sky Summoner are also very helpful. Krenko's Command can also help add more tokens while still triggering your Guttersnipe.
I currently don't have access to Nivmagnus, Talrand, or Krenko's Command.
Should I add another downsize and take out something?
I'm guessing that you haven't been playing for very long, because that much variety in a deck is usually a bad sign.
I haven't even touched a MTG Card before.