When you start your project the admin user role is subscribed to your "MyFirstModule" modulerole "User". Maybe some renamed that module and therefor it still is ascribed to it. So if you know for sure that the "admin" module-role has at least equal rights as the "user" module-role then only ascribing "admin" would do the trick indeed.
To use your example it might make sense to give the System.Admin User Role the "User" module role. Some modules, e.g. a "Notification" require all users to have a certain level of access, so that the users can see Notifcations in my example.
The answer is YES. The module roles at the System module cannot be altered ant thus I can be certain that the administrator module role has exactly the same rights as the User module role and thus ascribing only the administrator role would do the trick.
The reason the administrator has more access rights is due to the fact that the USERrole administrator is allowed to manage other userroles (All) As configured in the userrole.