Concerning the slideshow module, you have to check in the list of extension if you can share its content between website.
www.jms2win.com/component/content/article/135
If the extension can be shared the all the website where you will have shared it will be possible to manage them and display them in their websites.
Concerning "twitter" module or any module, by default their content is independent between website.
The only constrain concerning the installation of the extension in a slave site is that you have to install all the extension in the master before you "re-install" or share it in a slave site.
Concerning K2 again, if you want to share its content between website, this is possible.
As soon you share an extension between two websites, this mean that administrator or both website can change anything in this extension (as it is shared).
With JMS you share ALL the extension content or Nothing.
You can not decide to share partially an extensions.
Only the "Articles Sharing for JMS" provide the possibility to select the articles you want to display. But you can never modified an articles present in another websites.
JMS does not provide any ACL (Access Control List) faciliities for the extension for the extension and to control which peace of information can or can not managed by a usser.
The sharing factility proposed by JMS is done at MySQL level and consists in sharing MySQL tables. Therefore it has no actions on the records or subset of records present in a table.