Why does Turbo Man enter a supercharged car mode when hit with Thunder Bolt? Because it charges his motor and temporarily turns him into an hybrid electric car, which can run longer than with petrol alone.
Why were there two Plant Man on display in the Robot Museum? In Mega Man 6 Dr. Wily built robot duplicates of Centaur Man, Tomahawk Man, Yamato Man, and Knight Man, so it's entirely possible Wily built robot duplicates of 6's other Robot Masters (including Plant Man) as well, but did not use them.
Sound waves seem like an odd choice of weapon for a vampire-themed robot master, until you remember that bats use echolocation to help them see.
