AC21 and starting a company are totally unrelated. You can start a company any time, even on H1B. Once a company is started, it is supposed to be a separate legal entity. Working for this company is as good as working for any other company using AC21. However you should be careful that you work only in the capacity as mentioned in your labor application. I doubt if you mentioned the duties of a CEO or a company owner in your labor. Hence you will have to hire someone to do these jobs and work as a skilled worker for your own company under the CEO.