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Birds of a feather flock together. This is a universal truth and one that you ignore in a story to your peril. As story which ignores this truth may check a bunch of ideological conformity check boxes (and that never hurts in the market, as long as you pick the right ideology), but it tends to leave something false at the heart of the story which the reader will detect even if they applaud the ideological conformity. 

On the other hand, what makes birds "of a feather" can vary enormously. While birds, literally, flock with those of their own plumage, people can recognize commonalities with each other in all sorts of ways even if their external plumage is very different. 

In other words, a group of human can be diverse along axis X and homogeneous along axis Y. In fact, the more homogeneous they are along axis Y, and the more they differ from society at large along axis Y, the more tightly they will flock together despite enormous differences along axis X. 

You can see this in certain kinds of war stories, for instance. A group of soldiers who would never associate with each other in civilian life are formed into a tight cohesive unit who sacrifice for each other when they are cut off behind enemy lines. 

One of the reasons that this is a common literary trope is that it solves a problem for the novelist. Novels need conflict. But in real life, people who dislike each other avoid each other. To force them together, the novelist needs a device that forces them together, some point of commonality (serving in the same unit, being in love with the same woman, suffering from the same rare disease, etc.) that forces them repeatedly into each other's company. 

So, to bring a diverse cast together and make them stay together convincingly, you need to show that:

a) They are birds of a feather in some aspect of their lives. 

b) That they are unable to flock with other birds of similar plumage because some difference sets them apart from them. 

Or, to put it more simply, you have to convince us that these birds really would flock together, despite their dissimilar plumage. Accomplish that and you can make your cast as diverse as you like. Fail to accomplish it, and your story just won't be convincing, even to those who applaud its diversity on principle.