Well, if you want to do a small group 40g might me bit small, they don't need a lot of room to swim, as they swim in current, but some get more territorial and and need space. Now you can get one anthia without a problem and if you go this route I would recommend Bartlet's anthia as this is one of easiest anthias to keep. It might give a problem to your firefish though. Another one would be lyretail, about as easy as Bartlet, but gets bigger and more aggressive. If you want group of 3, try resplendent anthias, they are smaller and less aggressive, just make sure they have enough hiding spaces in your aquarium. All anthias are jumpers, so screen is a must, also at least couple of feedings a day. Those anthias I recommended to you will be ok on two feedings, other anthias need more frequent meals. You can train those to eat pellet food,I never had success getting them to eat flakes. Mostly I feed mine all kind of frozen food.