Hello,
For, Best practice to follow to create SEP clients group in SEPM ?
You can add the groups that contain computers based on the level of security or function the computers perform. For example, you should put computers with a higher level of security in one group, or a group of Mac computers in another group.
Use the following group structure as a basis:
See How you can structure groups.
See Adding a group.
Assuming all of the client has been all upgraded to the latest, does by moving client from one group to another cause any unwanted system restart ?
No it would not require any unwanted System Restart.
I would recommend you these 3 Articles, which would answer all your Questions as it seems you are in creating the proper structure of Symantec Endpoint Protection server.
Planning the installation http://www.symantec.com/docs/HOWTO55061
Getting up and running on Symantec Endpoint Protection for the first time
http://www.symantec.com/docs/HOWTO55274
Network architecture considerations http://www.symantec.com/docs/HOWTO55114
Hope that helps!!