1) If air is totally removed in the space where two bodies of different masses are falling, then the air resistance faced by the bodies is zero. The acceleration due to gravity is the acceleration of the two bodies towards the ground. Same initial speed and same acceleration, so the bodies fall simultaneously.
When air is present the bodies face different air resistances (buoyancy forces), so their accelerations are different and less than g.
2) Acceleration due to gravity g = G Me /Re² where G = gravitational constant, Me is the mass of Earth and Re is radius of Earth. It comes to 9.8 m/sec². It does not depend on the mass.
g = weght /mass = gravitational force / mass
= GMm/d² / mass = G Me / Re²