Well since no one who has post advice prior to the recent post knows anything about African cichlids, here are a few things to think about.
One Pharaoh noted the slow growth of calvus compared to mbuna. They will outgrow and most likely kill the calvus. Second, leleupi are paring fish, not harem fish. A group in this tank will pair and drive off competition, perhaps even killing them. Now add in that the territory they wish to defend (have seen a pair dominate most of a 1200 will either result in a lot of aggression between your lelepui leading to some deaths and then of course aggression with your mbuna. I am not saying you cannot keep a few mbuna in with a tang tank (depends on the type) and I am also not saying that a few tangs will not survive with mbuna, but a large mixture of both can be difficult to manage. I would recommend listing a stock list of your current mbuna.
Now as per your examples... I have plenty of tanks setup all but one dedicated to African cichlids. I have a tang only tank, I have a mixed shellie tank, I have a mbuna tank, I have a mixed tang (like the kind you want) with peacocks. I have all these tanks because at one time I have one tank with all these different types of fish in them. I found out early it will not work long term. As mbuna sexually mature they can become terrible. I have kept many different species of mbuna, and breed them. I have also kept a community tank of mbuna. I have tried mixing compressiceps (closely related to calvus) with mbuna. I can tell you from direct experience it is not a great situation for the comps (in your case calvus). As for lelepui, I have kept them in many different tank setups. I have found a few things in my experience with them. One, they will pair. Two, anything in the pairs way will be killed or pushed a side. Three, mixing lelepui and other similar shaped Neolamps will result in a fight to the death usually between the males. (my case was cylindericus).
On the note of any who chimes in with, but I have kept them without problems. I do not doubt you. However, which tangs, and which mbuna will make a difference. Also as we all know one fish is not the same as another fish. You can have a peaceful fish that will dominate a tank, and an "aggressive" fish that will be bullied to death. It all depends. Also we have to think about when fish were added. There is a world of thought that if fish are raised together from little fish it will mean no problems. I highly doubt this thought. I think people have seen too many discovery shows with apes and kittens or dogs and cats. Fish are different. They are not domesticated. They are what they are. If a fish gets hungry enough, it does not matter if it has been in the same tank as another fish from 10 years, it will eat it.
Now, if you still want to try this set, and you have thought about what everyone has added, and then do it. I would not tell a person what they should do. The forum is for advice, or to give my experiences. I would however, recommend being ready to lose fish, or return fish. Mixing lakes and mixing types of fish require people to be ready for one of those two things.
GL