Assigning git branches to agents/pools

I'm not sure if this is the best configuration, so I'm open to being told how to do this properly.  We have started rolling out TeamCity with a PHP project with git for the VCS; we're currently running PHP 5.6, and want to start testing on PHP 7.0.  I have an agent set up with PHP 5.6, and another with PHP 7.0 - all of our build projects are assigned to the PHP56 agent pool currently, and we want to start running builds against PHP70, but we're pretty sure we're gonna need to make code changes, so we had planned on branching, and were hoping to assign the branch for each project to the PHP70 agent pool while developing and testing, and then moving the main branch over once we're done.  Unfortunately, I can't find a way to assign a branch to an agent pool - has someone else already done this, and might know how to achieve this?

Cheers

Ronan