A simple video mixer will not do. My guess is that your cameras are incapable of locking sync to an external signal. The switcher will have to have built in 'Frame Synchronizers', which will put all the cameras in sync so that fades and clean cuts can happen. Get your gear ahead of time to allow for a changeout if you find something just doesn't work with something else.

Bet that multi camera shoot is looking more like you will do it in editiing rather than live.