It's quite possible but imho by the start of the game it wouldn't really be that realistic.
Delhi had been controlled by muslims for quite a while by the start of the game and was firmly entrenched in the indian political climate. I'd also argue that the various muslim states in India shouldn't be in another tech group than the hindu ones as most hindu empires borrowed just as heavily from arabic and persian influences as the indian sultanates did. The first 50 years of the game is supposed to be the Vijayanagara empire's most successful period. A success heavily dependent on using muslim military tactics and the import of horses from arab merchants (due to the climate any south indian empire would need to rely on imported war horses, this was true for the Bahmanids to the north as well).
Also considering that the starting year of the game is literally Delhis darkest hour when they where basically in control of less land than you could see from a tower in the city itself I'd say any boosting of Delhis already a-historically strong position in vanilla might warrant some extra consideration.
The Indian sultanates are too weak in vanilla but this is because of the way religions are handled imho.
It's your game though so please disregard these opinions if you want to
You are right that separating unit_type from tech group during setup allows for some nice customization and as others noted all you need to do is add a "unit_type =" entry in the history file of for instance the Delhi sultanate (I believe this is already done in the Russian country history file in vanilla HTTT if you want to look at a file where this occurs).
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