Sorry but I don't think it is a good mix. Your Tropheus are vegetarians but they don't know it. They will eat anything offered or found in the tank! Your Frontosa is a opportunistic feeder, they do need more protein then Tropheus, but also enjoy veggie foods. N. brichardi is also a meat eater. You have a chance to compromise health of your Tropheus while insuring that everyone else is getting enough food. Tropheus also do better in large groups all of one kind in larger tanks. If you were to remove the Tropheus, then I like the idea of a small group of N. brichardi with your Frontosa.
Your tank is on the smaller side and Frontosa can get quite large, so that is going to limit your choice of tankmates. It comes down to which fish do you want more, and then you build your tank around that species.