The external tanks on the Russian tanks are diesel, not quite so volatile as gasoline. In fact under certain circumstances it can be difficult to ignite, although if the tanks were to make them especially vulnerable I very much doubt they would design them that way...further than that I cannot comment on Russian design beliefs.
I am an OLD tanker so my info is somewhat dated, even though I try to keep abreast of tank design. The idea of some lone trooper placing a rock to stop a tank is "movie stuff", not going to happen. An immobilized tank is not dead, only immobilized, but still fighting...time and circumstance may change things. A blown end connector would immobilize a tank, but they are hard to hit.
Stands to reason, the DOD wouldn't be paying $ 6, 200,000 for each tank if they were easy to neutralize. The demise of the tank on the battlefield has been predicted since WW2...but weaponry has been a continuous .."offensive punch..followed by defensive counter..the cycle repeating itself with regularity.
Molotov cocktail.. a bottle nearly filled with gasoline and a wick of gas soaked cloth hanging from the neck down the side...light it..throw it. Works best with "jellied" gasoline, which can be accomplished by adding natural soap (not detergent) to gasoline..gives a napalm effect. These worked in decades past..if the trooper could get close enough...but not so good with newer extinguishing/defensive systems. Likely there are now vid-cams located where even the smallest corner of the tank is exposed and weapons where every spot can reach. Besides, tanks are always accompanied by infantry and other tanks.