You are confusing the Mandalorian strategy. They go in, pillage and burn, and take the survivors as slaves or soldiers. And if they can't, they burn it down. The Sith Empire sought conquest, the Mandalorians sought battle and victory.
If that's the case, I would think even more that the Republic would have stopped them in the Core Worlds if they would even try to raid them. You can't destroy city worlds this way (the Mandalorians actually occupied Taris and the Tarisians formed a resistance until the planet was freed by Republic forces).