After you make the maps at the same scale, you could just use CS:GO engine and just sync the movement on the others.
But, doing this the user experience in CS:S and CS1.6 will be similar to CS:GO physics (not mentioning lag or other issues).
By now I'm sure everybody has CS1.6 and CS:GO was on more then 8 sales. Buy the damn thing already.
P.S. nobody cares about CS:S