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With an EAD, you can work anywhere until your EAD expires. Whether you can open your own business may be depending on different state laws. You do not need an EAD card to open a business. However, in order to work for you own company and get paid by your own company, you will need either EAD or H1B.
If you get laid off and your sponsoring company does not withdraw your I-140 within 180 days after you filed your I-485, based on AC21 you should be able to get a new job in similar field and get your I-485 approved with your job. In this case, you can not be working in Pizza Hut unless your previous job was working at a fast food chain and your precious employer filed yoru Labor Certification based on that job. However, INS has yet to enact regulations on this law.