For best performance you should probably keep the maximum operating frequency of a 555 below 1MHz.
The current required to drive a MOSFET is determined by its gate capacitance, which is given on the data sheet.
For a given switching speed the approximate formula is I = (V*C)/t where V it the switching gate voltage (typically 10V), C is the gate capacitance, and t is the desired switching time.
If the gate switching charge is given on the data sheet, then the formula is I = q/t.