I don't have much experience with the g2 so I'm probably not the person to ask but yes you can use just mono as well. However I've been hearing from a bunch of people this trick with lead and mono works well.
Possible with just mono, but it can difficult and maybe frustating. I wouldn't recomend it without a proper release and drag set up, in development stages now. Will be available next spring, it will be only a small modification and 2 second swap over, different release/drag.
I would buy one now, use it with mono and lead core combination (super set up) and very easy to use and ctaches fish. Get used to it, then next spring pick up the new style release, work it out with the G2 people.