Thank you David , that is a really interesting post.
Originally the common marmoset isn't native to Minas but only to the North-Eastern region of the country. The black eared marmoset is on the other hand native to the State but has spread as an invasive species (historically and up to the present) due to the illegal pet trade. Neither of these species are native to the Atlantic rainforest ecosystem but originally inhabited only the Cerrado (black eared marmoset) and Caatinga (common marmoset) respectively.
The marmoset species native to Minas are the buffy tufted marmoset, the buffy headed marmoset and to a far lesser extent the Geoffroy's marmoset (which is also present as an invasive). However, the Geoffroy's was only really found originally in areas of Minas that border the state of Espirito Santo and Bahia but then spread as a result of the pet trade too.
My Mineiro colleagues are really struggling with the situation of the spread of these invasive species into protected areas and the scale of hybridization in the state (and in other states with the buffy tufted and buffy headed marmosets) is really alarming and the risk posed is genetic extinction of the native species.
Here in São Paulo they are also to be seen all over the city and the interior of the state and when I go to protected areas or even just parks I almost always see these animals (I've seen them wild in Sorocaba zoo too and ironically once saw females interacting with a captive male buffy tufted marmoset with mating receptive behaviour). The problem is also very present in Rio in both the city and state and like what you describe in BH they seem to be in most of the trees in urban areas.
Even so I have to admit that even though the species is such a huge problem for conservation I really do deeply admire the resilience and adaptability of the common marmoset

and have a grudging respect for them. They are such a tiny monkey but they are plucky and true survivors (unfortunately for their close relatives) like little cangaceiros.