When the Soviets(Russians) invaded Afghanistan - and Pakistan supported the resistence - how much did the Soviets attempt to attack Pakistan ? Minor aerial incurisions - they never directly challenged Pakistan and Pakistan didnot have the protection of NATO or nuclear weapons at that time.
So - the question is - are the Russian prepared to directly challenge a NATO country - i suspect the answer will be - no. Time will tell .
Pakistan is slightly a bigger fish than Romania or Slovakia. And Pakistan had China.
I think Russia will not allow it without serious consequences. If nato is united it will be ok. If they shy away from protecting weaker members its going to be bad for all.