Flocculencio
Donor
I was thinking less as partners of the Portuguese within Portuguese territory and more as neighbors and allies. The Portuguese have their own territories and a small polity next door is claimed and ruled by the StTCs, the Portuguese accept them as handy allies who are heretics rather than heathens and pagans.
Edit: sorry Flocc, I didn't realise it was you.
Hello!
My gut feeling is that this isn't particularly practical. Catholicism did have a genuine base among the lower castes (as testified by the large numbers who converted to it- Roman Catholics in Kerala outnumber all the St Thomas Christian factions put together), and the Portuguese were actively hostile towards the Surianis (StTCs), even though through doing so they were alienating a population with a pivotal economic role and undermining their own trade position. Why give concessions to an economically powerful but hostile minority when you have a larger group, specifically beholden to you, to work with?
IOTL the Portuguese don't seem to have made much political use of the converts and the Suriani seem to have actively worked with local Hindu aristocracies to undermine Portuguese dominance (leading to a gradual ebbing of Portuguese influence in Kerala).
If we're considering alternative courses of action, the path of least resistance, so to speak, is to rely more on the converts, rather than on doing a 180-degree turn on your attitude towards the Suriani.