I'd be curious to see the data on this. Are ethnically homegenous countries psychologically homegenous as well?
Not entirely. Class divisions will still occur, even if they aren't on ethnic lines. Gender and sexuality differences still occur.
And how does this psychological diversity manifest in racial groups - is it innate or cultured?
Cultured. To think that it is innate is a massively racist concept.
If it's cultured, then I would expect it would fade through assimilation. So, if this theory is true, a third-generation Asian-American wouldn't offer as much psychological diversity to a development team as a first or second generation immigrant.
This is only true if actual assimilation happens. It is pretty clear that the US, for example, hasn't really assimilated its African American population, over several centuries.
And if it's cultured, wouldn't a first-generation Polish immigrant offer greater diversity than a second-generation Asian?
It isn't like we have a concrete measure for this. And you seem to focused on strict "more or less," when with diveristy, you are concerned about different. Both those candidates would provide different outlooks.