Stainless steel is not a good rotor material because it's not a conductor of magnetic flux. You want iron or steel for your rotor material. Also, the reason that folks here tend to use rather thick rotors is because the Neo magnets are so strong that the flux cannot be fully conducted by thin (sheet) metal. Some people have had success using circular saw blades with smaller magnets.
Another factor is mechanical strength. A ring of Neo magnets will exert an incredible force between two rotors (in a dual rotor system). Also, you want really small clearances to the stator for air core alternators. Any rotor flex during yawing will crash the magnets into your carefully wound coils.
Inertia really isn't an issue for most applications. The inertia of the blades will be much higher in most cases and inertia tend to dampen vibration anyway. Where weight becomes an issue is heaving it up into the air and keeping the tower straight!
As to having a lip on the edge to contain the magnets: Some folks using brake drums turn them down except for a small lip at the edge for the magnets to sit against. Other people use dowels pins or simply rely on epoxy or other adhesives combined with the magnetic force to keep them in place. Assuming the lip is magnetically conductive, the rule seems to be to keep the lip lower than about 1/4 of the magnet thickness.
