I think if you were to add any more users, I think you'd be unable to use Google Hangouts. Max number of people that can be in a Google Hangout at any time is ten. I'm not sure if there's a hard limit with WebRTC, though you might reach a critical mass with how many people display on screen. As for keeping text chat from collapsing into a neutron star, use inline rolls whenever possible, turn off dice images, and if you're really in a pinch - turn off character portraits. Also go into your campaign details and delete the chat log on a regular basis.