Can I truncate users table?
No, you can't truncate a table and leave data in it at the same time. That would be a contradiction in terms.

You'll need to do a standard delete

DELETE FROM users WHERE uid NOT IN (0, 1)


(it's important to make sure the anonymous user, ID 0, remains there too).

If you use the direct approach, be 100% sure that you know what you're doing. Unless you know different, there will probably be related data that doesn't get cleared out by simply removing an entry from the users table. 

The best way to delete users is through the API: user_delete_multiple()