I think that in the first debate, every candidate who is on enough ballots to win should be included in the debate.
After that first debate, you should need 15% (or mayber 10%) popular vote support to be in the other ones.
But then how do we decide 10%? Which poll do we use?
In 2000, I think the debate commission already decided to use an average of five different polls...or maybe you just had to hit 15% on one of the five, but they used more than one poll.
You can't have six candidates in every debate...look at the Democratic primary debates. Including Sharpton and Kucinich early on was fine, but at some point they were just distracting and eating time.