* You can do this on the OPT *. Without OPT F-1 employment rules are strict. But the OPT is basically a green card when it comes to what kind of work you can do (anywhere, you don't have to ask anyone's permission to change employers, you can freelance, you can start a business, you can work for multiple employers).

You need to work full-time on OPT. I don't know how they check but you need to be prepared to prove that you're working full time. (I know about this because if you're freelancing, my school told that I need to keep extensive paper trail to prove that I'm employed.) There's limitations about how long you can be unemployed in the OPT.

You need to register your company in E-verify for OPT extension. To register for e-verify you need tax ID numbers and a DUNS number as well. It's a pain but I was able to do it myself. (They make you study for a quiz, etc). You actually don't need to e-verify yourself. Just registering the company and getting an e-verify number was enough for us.