People assume the problem with a pet in a Mumbai flat is space. In practice the animal cares far less about floor area than about whether anyone is in the room. The real constraint is storage.
Three things that need a home
Where the food is kept, sealed and out of the humidity. Where the carrier lives when it is not in use - it is bulky and you will need it. And where a wet animal dries off in August. Solve those three and the rest is easier than you expect.
One spot that is unambiguously its own
However small the flat, the animal needs somewhere that is not a corridor and not contested. A cat needs height for this; a dog needs a corner out of the main walkway. An animal without one never fully settles.
Density is an advantage
A pet raised in this city grows up used to lifts, noise, strangers and close neighbours, and is calmer for it. An animal kept away from all of that is the one that struggles later.
Weight
It is easy to overfeed an animal that cannot run much. Portion by your vet's target weight rather than by what it asks for, and adjust monthly.