I've written about this in the past here. More to the point, it would depend. If they were domiciled British Sikhs, then they would more than likely be in a regular British regiment because they would not be considered colonial. The film does depict the Sepoys as being specifically from an Indian regiment, but it is not clear whether or not the soldiers are meant to be part of the regiment (the white, British soldiers in the scene appears to see them as strangers) or simply replacements being shipped to another part of the front. It is likely the latter, however.