I have the official document from Chen Dan Hong attorney, they are really nice and there is online chatting function on their web site. That's the place I got the document.
Yes, You can start working right after USCIS receive your petition based on the H1-B portability from a cap-exemp to a cap-subject H1-B. Don't worry!But I think this year you better wait till you are sure you get the visa.
If you need the document, you can go to their on line chatting to request or leave your email address here. I can send it to you.