Can models be asked for anything?
You can definitely share all your fantasies with the models as soon as you access a private show. It is very rare that a SexCam website would allow you to publicly list your sexual wishes. Models have the right to refuse a private session and even specific user's demands. Most of the camgirls and camboys are familiar with their job and they don't have any taboo, although it might me possible that some of the demands will be judged inconvenient for the models. In those cases you could always look for another model. Keep in mind that most of those websites have different and very organized categories : an easy way to quickly target your wishes.
In any case, respect for the models is one of the most important rules in the SexCam websites.