RC time constant and delay
So as long as RC for different values of R,C are equal, delays are
  equal, is this right? For example R = 8 Ohm, C = 0.25p, RC = 2 and R =
  5 Ohm, C =0.4p, RC=2


In theory yes but in practice no. The output driver has a finite output impedance that will add to the external series resistor and as that external resistor value gets lower in value it will be swamped by the residual output resistance of the driver.

This output resistance might be 10 ohms to 100 ohms hence it will swamp the values of 8 ohm and 5 ohm you used.