We have the master site,
From what i read,we can create a new site slave,
with only a few modules and plugins of the master site.
1)In the slave site can we install a new plugin ?
Or we have to do it in the master one ?
2)If someones registers to the slave site he becomes member of the Master too ?
3) Any hacks in the core ?
4) Master and slave sites can have a common login ?
a single login/out for all of them ?

Thank you

Is is working this site ?
5 days no response to make a sale,
support how it will be ?

Sorry for the delay in the answer.

0) The concept of JMS is based on the sharing
- Sharing of the same Joomla application (php code)
- Sharing of the content of a limited number of extensions

1) Yes you can install an extension in a slave site if it already exists in the master with exactly the same version. (Because you are sharing the same PHP code for all the sites).
All the extension must be installed in the master before they are re-installed in the slave site or using the JMS Tool to install them in the slave site or using the Joomla extension manager / discover menu to re-install them in a slave.

2) This depends if you decide to share the Joomla users between the websites.
When you share the Joomla users (DB content) then yes the user is automatically shared (available) in the other site. This does not mean that master but the website where you ahve decided to physically store the user. This can be the master or any other website that you will have created to derive the slave site. (ie. a Portal website).

3) Yes JMS perform hack of the core of Joomla.

4) Item as (2).
If you decide to share the user, then yes. Otherwise No.
When you decided to share the users and that you want to remain logged when you change of website, this is possible with website using the same domain name (subdirectories or subdomain). In the case of different domains, you will also need the "Single Sign-In for domains" extension.
www.jms2win.com/en/joomla-multisite?page...51&category_id=1

5) Support priority is always given to the billable support that can be ordered at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1

Free support is give via this forum and sometime, this may sometimes require several days before to receive an answer.