The problem is the bacteria adjust to the amount of fish you have in the tank. So right now, unless you just cycled with ammonia, your bacterial can support 7 fish. Swapping the 2 zebras and the one tret (3 fish) for 6 cobalts is an increase of 3 fish, but it is safe.
The cobalts are your most aggressive fish with 2 already in the tank, and you need 8 of them to manage aggression among them.
See if you get any ammonia or nitrite in 3 weeks, and then consider adding 6 yellow labs. Wait another month and add 14 maingano (not johannii...cyaneorhabdos).
Since you already have those 3 one assumes they are your favorites. A month after you add the maingano, maybe a blue barred fish like Cynotilapia zebroides Jalo Reef.