The available ninja list lets you see ninjas that are listed as available. Not all of them have been distributed yet. About different forms, you can see them through "Train" interface, by clicking "?" in Upgrade stars tab. This will show you whether or not ninja transforms during star upgrade. The only downside is that you can't see ninjas you don't have unlocked, but you can go to our simulator site (top of the page, on orange bar, there is "Forum" "Fans" and "Simulator". There is a list of all currently available ninja there :)