Can’t you just exclude the princely states
Couldn’t you just bring them in from Muslim majority areas
I suppose so, on both counts. It could be done, and certainly its not as if the Princely states are going to be going about this themselves, but the question has to be why. The idea of colonial divide and rule ethnic favoritism was much more of an African phenomenon than something practiced in Asia or Latin America, so this would be a new experience. The population of direct rule regions of the Raj was absolutely massive, so it is not as if there are a shortage of Muslims. The issue comes in the form of the question of why Muslims would be used to administer, say, rural areas of the Madras Presidency.
It was the case that the use of British administrators, particularly after the 1858 revolt, was a salve on ethnic tensions because, while not in any way unbiased, their forms of bias were different and didn't step on the wedge of Hindu/Islam which was so utterly explosive. Perhaps more important was the institution of the army.
The model of divide and rule on the basis of ethnic differences was much more applicable in smaller population groups with a lower level of political, social, and economic development. This did not significantly apply to India. There was no need to impose caste systems based on ethno-religious differences on the population to facilitate rule: India already came equipped with a literal caste system. The wealth of the Muslim areas of India and the Hindu areas of India were not really all that different at a major level.