This is a fine place to post your question
Unified management is definitely the way to go here, it will make your life simpler in the end. There's a couple of approaches you can take in terms of building policy:
1. One policy to rule them all. Create a single policy that applies to all your gateways. You can create gateway-specific rules that are only applied on specific gateways (using the Install-On column). This is not an approach I see regularly and I think it makes the policy more complicated overall. It's not what I would recommend.
2. Create a different policy package for your Branch Offices. If you want to use your existing policy as a basis, you can do a File > Save As and create a new Policy Package (after saving other changes you might have made). One thing you definitely want to do to ensure the wrong policy isn't installed on the wrong gateways is go to Policy > Policy Package Installation Targets and specify the specific gateways the Policy Package applies to.
The downside to #2 is that you will have to duplicate rules (particularly App Control/URL Filtering rules) across your two policy packages, but the resulting policies should be simpler to manage.
When you upgrade to R80.10+, you can have both policies use the same App Control/URL Filtering rules using the same inline layer.
I realize this will probably result in more questions, but it should be enough to get you started.