It's recommend that the Connection Server and Horizon Agent be at the same version. In addition the Connection Server can be newer than the Horizon Agent but you don't want a newer Horizon Agent then Connection Server (There have been a number of issues when doing that). There are cases where a slightly newer Horizon Agent might be recommended by support to fix an issue but it's best to upgrade the Connection Server(s) as soon as possible. Why not just upgrade to 7.8 for both the Connection Server and Horizon Agent (Assuming this is a supported path)?