Dear Nathalie, concerning the content sharing, JMS does not share the content as explained in the FAQ
/faq#sec-72
JMS does not share the user, forum, new, articles, or whatever that is stored in DB.
There is a specific component that we call "Articles Sharing for JMS" that allow only display articles coming from any websites defined in JMS.
For more detail on this component, you can go on
/download?page=shop.product_details&flypage=flypage.tpl&product_id=34&category_id=1
Concerning jos_ table prefix, you can continue use it on your master website but we suggest to use another prefix. If this is an existing website you can continue to use it.
The risk to keep jos_ table prefix concern the problem that you could by error destroy it when you create a new slave site and when you don't take care about the table prefix.
In this case, by default Joomla use the jos_ table prefix and will backup the current one in bak_. The result will be that your master website will be destroyed and you will have to restore the bak_ into jos_.
If you want to keep jos_ for your master website, to reduce the risk to destroy it, we recommend to use another DB for the slave and never use the jos_ table prefix. If by error you create a jos_ table prefix, this will not destroy anything.
Read the chapter 3.2 of the user manual that present the possible architecture.