I think you will be in a far better position to tweak and adjust classes after you have run a season. I think what many don't realize is that you don't create a bunch of classes based on who you want to come out, but rather try to anticipate who and what are coming, and the most logical way to split them into competitive classes. The logical time to create more classes is with groups that are large enough to probably require being split up anyways, and classes can make the splits more closely competitive than a random split. I guess thats the beauty of non-points classes is that you can tweak, adjust, create or eliminate them based on the day's participants.
I've seen classes created that consistanly have little turnout. I've also seen specific classes created that those who were eligible ran other classes because they wanted to run a classs where they had some competition.
I don't know about bikes, but I can say that when it comes to mini quads, there are nearly as many girls as boys, and they are every bit as competitive, if not more so. I doubt most of those girls would not want to be kicking boy butt. I guess polling some of those parents is tehb best way to find out.
As for senior quad, from my own experience, is that older racers, while most are highly skilled and experienced, they fear racing with young riders who do not have the same fear of injuries ond very long recovery times that realistic older guys have. Younger racers are far more aggressive and bigger risk takers in general, so for most senior quad racers, its a matter of safety over any issues about fairness in scoring. Senior quad classes tend to be some of the largest quad classes now, so if quad classes are large enough, its a logical place to make a split. My only personal difference with most places is that the age should be lifted from 30 to 35.