The panel I have now is a 100amp main breaker and I still want the ability to keep some of the circuits that it currently has in it. I am putting the 200amp main lug in my house and was wondering if it is safe to put a 100amp break in my main panel to feed the 200 amp main lug panel and if so then what size wire should I use to do so? The reason I am putting the 200 amp panel is to possibly upgrade the system to a 200Amp.

Yes, you can use a 100 Amp breaker to feed a 200 Amp panel, obviously your limited to the 100 Amp.

For this purpose you can use #2 aluminum wire.