I imagined the Greeks (and their Indian wives) living in their own communities, feeling threatened by the Indian majority. So, they side with who is culturally and religiously similar to they are, same way Ethiopia sided with Europe against the Ottomans and their allies.
Then, Alex’s main army settles in India. OTL, Alex’s conquest was enough to Hellenize somewhere. Though the problem is that there are only two real opportunities for us to get a significant number of Greeks in India.
1) Why? Why would they join the conqueror, particularly when this conqueror has had absolutely no problem with killing Greeks loyal to Persia or suspected of being loyal to Persia or Greeks who he just happens to be paranoid about?
There's no reason for them to see this barbarian warlord as on their side more than the other Indians (used as "inhabitants of the subcontinent") If they have Indian wives, that just makes it worse - for Alexander and anyone else who thinks they'd say "Hey, fellow Greeks! Welcome!"
2) There is no way in hell Alex's main army is settling in India.