I thought this might be of interest to the discussion.
I was just doing a bit of research about the mixing of troops from different communities. Troops in certain Presidencies, in certain regiments and at certain times were mixed by religion and caste. Also, the approach in other units of not mixing troops came not from the fact that they could not be mixed, but rather it was contrary to imperial objectives to mix them.
The 1859 Peel Commission noted that the Madras and Bombay armies that had ignored or undercut caste and religious distinctions had not risen up and so recommended that "The Native Army should be composed of different nationalities and castes and, as a general rule, mixed promiscuously through each regiment."
By the 1880s, the army had gone the other way there were by this time a number of single caste or single religion regiments and regiments separating castes or religions by company. A number of officers opposed this approach, but it was championed by Lord Roberts, reflecting his martial races theory and experience of serving only in the Bengal Army. This was a British legitimation of existing cultural distinctions based upon Victorian theories and discourse about the importance of heredity and biological determinism. Such a discourse was important for justifying and legitimating colonialism, both for British and Indian audiences. It also provided a justification for reducing recruitment from the communities and castes that had taken part in the 1857 uprising. (Philip Mason, A Matter of Honour, and Richard Gabriel Fox, Lions of the Punjab)
There was also a crucial "divide and rule" aspect to this separation, as the Punjab Committee noted, "“As we cannot do without a large native army in India, our main object is to make that army safe; and next to the grand counterpoise of a sufficient European force comes the counterpoise of Natives against Natives. [...]To preserve that distinctiveness which is so valuable and which, while it lasts, makes the Muhammedan of one country despise, fear or dislike the Muhammedan of another, corps should in future be provincial, and adhere to the geographical limits within which differences and rivalries are strongly marked. [...] By the system thus indicated two great evils are avoided; firstly that community of feeling throughout the native army, and that mischievous political activity and intrigue which results from association with other races and travel in Indian provinces.”
Roberts also applied his theory of martial races to Highlanders, as well as Gurkhas and Sikhs. His reports disproportionately focused on and praised the actions by and connections between these groups and became reflected in the popular narrative (eg press, written by officers acting as correspondents or fiction, eg Kipling's Drums Fore and Aft), helping create links between these groups and furthering the popular myths. As a private of the Derbyshire Regiment said, "'It's no use an English regiment trying to get on where there's a regiment with the kilts.' Although the private referred to a Highland regiment, had he been an [Indian] soldier he might have expressed similar sentiments about Gurkha or Sikh regiments [...]" (Streets, Martial Races).
Sorry, that was a bit long.