They did destroy Tatooine...
But how much lives that took? Well we have no idea, we don't know what the infinite empire did. Still the greatest fight ever, probably between Ancient Sith Empire (Ragnos, Sadow) against the Infinite Empire. Two great powers filled with force users, now thats going to be a war worth seeing.
350 Trillion probably isn't that much when you look at the entire Galaxy.
Malak destroyed one planet and killed billions if not tens of billions. Do that a few hundred times and you have your trillions. The damage to the galaxy would be extreme though and it would take a very long time to recover. And the republic would probably fall anyways.
The number of causulaties that the Yuuzhan Vong inflicted is extremly high, but not unreasonable, considering they were trying to make humans extinct. Thye killed the eqeivlent of about 60,000 Earth-populated-equal worlds. There are about a million member planets in the Republic I believe, and about another 50 million colony worlds. Most planets in SW are less populated though. Dantooine I think only had about 2 million worlds. Tatooine had about 200,000 people. In the attack on Coruscant, they probably killed at least 500,000,000,000 people.