
I'd be cautious about having multiple firewall settings floating around. It won't apply to workstations in the same OU if they are not in the group as well. Please note that as long as only the group you created is in the scope it will only apply to the workstations in that group.

You'll want to make sure this GPO is linked to the correct OU. Then create a GPO with the desired firewall settings and remove "Authenticated Users" from the scope of this GPO and add the group you just created. If you want to apply certain firewall settings to only a few workstations I would create a group which has for members each workstation you want this policy to apply to.

GPO: "Computer Configuration/Administrative Templates/Network/Network Connections/Windows Firewall/(Domain Profile or Standard Profile)/Windows Firewall: Protect All Network Connections" - read the description.
