I think it better to keep the nuclear retaliation doctrine simple and clearcut, which it is. If Pak goes nuclear, whatever the size and range of the weapon launched, they will be wiped out. No need for further complications.
That is insufficient deterrence to Pakistan going nuclear, which is my whole point. Since Pakistan gained nuclear weapons, India has never been able to cross LoC/IB, we sugarcoated our inability to do that with "respect for International law" but we know that's bullshit. When they didn't have nukes, we crossed borders and wiped the floor with them in 1971.
On the other hand, Pakistan has been able to breach LoC/IB, even after both countries gained nukes. Whether it's through their regular army in 1999, or through non-state actors (an oxymoron). Ever wondered why they could do it and we couldn't? Because this is a conflict where Pakistan is counting on India's greater stakes (more cities/population/infrastructure/) coupled with a government that's genuinely answerable to the people, to translate into a crippling amount of self-restraint, which Pakistan does not have.
Apart from lacking self-restraint, we know from Kargil that the Pakistani Army is essentially a Junta of warlords, of which individuals plan out high-level offensive action without consulting with the rest of the brass or the government, and who don't particularly have a high IQ (we now know this from just how poorly planned Kargil was, from their side). So we're dealing with low-IQ people with a God-complex, who have little to lose or answer for, and who if not defeat/conquer India, are perfectly satisfied with blind retribution for their imaginary "injustices."
They will go nuclear the moment we cross the LoC/IB, and they will continue to cross LoC/IB themselves without any tangible consequences, until we raise their stakes.
One way to raise their stakes is to have an end-of-the-world doctrine installed, like Israel. Israel thinks that when hordes of Arab armies are marching down its borders, it will have to undertake one last spectacular nuclear launch (possibly 400-500 warheads, maybe more), which sends the Muslim world back to the 1800s. There are also elements of scorched-earth to this doctrine. In addition to nuking capitals, cities, it also targets oilfields, known coalfields, cultural/religious sites, and more. This presents a very scary prospect for those wanting to march on Israel. Since it's an internal doctrine, there's nothing official about it, and it's never up for public debate, and the Israeli state denies it.
Make no mistake, the Pakistani first strike will be quantitative. They wouldn't want a limited exchange that causes us to back off with just one retaliatory second-strike, against one target. They will unload their arsenal on us. That's why we need a cataclysmic second-strike that extends well beyond Pakistan, similar to Israel, so that the decision to strike India becomes costlier to Pakis, to begin with.
China is not on the list because they're a waste of good warheads, which we'd much rather lob on targets in the middle-east.