Flew into Houston once from overseas long ago, got hung up for at least an hour, my ride left before I got out. I'm sure things have changed since then.

Why not try to email or phone somebody at the airport, they might be able to connect you to someone in the customs area, or even a ticket agent who is familiar with what kind of layover time should be allowed for.

start here

As for the advice on rail or bus, yes those are fascinating ways to travel, but isn't it going to be a $30 cab ride to get to a scurfy bus station downtown from the airport? Or have they recently developed some intermodal centers to connect between air and ground?