Question about multisite

Dear friends,
(1)In case of multi site do I need to host all sites in the same server?My network will contain 6 sites -all are separate sites just logins will be the same.
(2)I will only allow users to create blogs on the root site,not on the other 5 separate sites ,how is it possible?I also want to keep the sub domain/sub directory blogs creation feature under myself .I like to enable the feature for the interested selected people only,I don't want all mass create the blogs theme-selves ,how to control it?

(3) As I said I will include total 6 separate sites into the network,how to merge the 2 existing sites (I already built) into the network ?