Palpatine's arrest is, in fact, not legal. What Mace did was treason against the chancellor. Sith or not sith. Of course, when you find the Sith that has been orchestrating a war for years and years, you kind of want to go to his office at the moment you find him out. As for a deeper plan of attack, I believe at this one point of all Mace Windu's time as a Jedi Grand Master his mind was clouded. He had one objective and one only, to kill the Sith lord. As for reinforcements, whose he going to bring? Clone troopers? If he brought more jedi you can bet he would have slaughtered them like the rest. not even Saesee Tiin, the temple's "Most celebrated swordsman" stood 5 seconds against Palpatine.