Sonic.net

I'd like to be able to let visitors to my house use my network, but limit them to only using (collectively), say, 2048k down, 384k up, and 1024 concurrent connections.

When I was using the old Zyxel (and, before that, whatever my previous ISP used) bridged to an OpenWRT router, I was able to configure my router to do what I wanted. (It wasn't easy, but once I got it working, it worked.)

Is there any way to do the same thing with the Pace?

No. Connect your old router to your Pace as though the Pace is your WAN and dedicate it as a guest AP.