It is posible to have a 100 breaker for A/C, it's very common around here to have a 100 amp breaker serving a disconnect for 3 or 4 A/C units rather than having 3 or 4 diferent home runs for each unit.
To answer the guy's original question, yes it is posible, but if the A/C's draws a combined load anywhere near that 100 amps, it's too much for the panel with all the olther stuff.
