Can I post the same photos on other royalty free sites once I've posted them to my Adobe account?

Yes you can. You should read the instructions. It's well documented, with a lot of other usefull inf.ormation.

If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

Yes. But you need to make sure the other sites allow you to do so.