Both sides have to agree to the "limited" part for it to stay limited. Mind you, threats of nuclear escalation are routine from Russia.
It’s more that each side has to moderate itself. That’s true if both have nukes or just one. And it’s much easier to moderate oneself when it’s in one’s own interest, e.g. to avoid getting nuked.
If you mean a scenario where Russia uses its entire arsenal at once, I think I would agree — but that would leave them extremely vulnerable to everyone else. Even without that, between the shared border and the nuclear reactors, lots of fallout would end up in Russian soil, in their farms and food.
For the size of nuclear strike that would allow Russia to not hurt itself directly by the attack:
If Ukraine found a some still-functioning nukes lying around in a formerly-locked Soviet-era cupboard, what would stop them from being so already with current weapons?
Russia doesn't seem to be able to prevent conventional strikes by Ukrainian forces. Is that only due to US assistance with materiel and intel?
Ukraine isn’t in and probably never will be in a position to credibly threaten a MAD second strike to Russia. From number of warheads to strategic depth, it’s not a realistic option. Instead it would seek to credibly threaten limited nuclear war using tactical nukes.
This would only make sense with European support. And at that point, it almost makes more sense for Europe to directly enter the fight. (Or give Kyiv the nuclear weapons.)