Allowing restricted accounts to use gmail/hangouts
The main difference between a "user profile" and a "restricted profile" is that a restricted profile doesn't have its own Google account, which is needed for Gmail and Google+ Hangouts. A restricted profile with a Google account in it is just a normal user account.

If you want to restrict access to features in a normal user profile, you need to use device-administrator software to impose a security policy. This will also allow you to remotely/centrally manage each device.