How to scale down voltage of the the video signal without affecting input wire impedance
Many triple video buffer ICs are available. These generally have a voltage gain of +2 to compensate for loss in the external output resistor when terminated. One example is the THS7314, which can handle up to 1.4Vpp on each input. 1V in produces 2V out. To divide this down to 0.7V you need a divider ratio of 2.0/0.7 = 2.86, so in the circuit below R1 should be (2.86-1) = 1.86*R2. The output impedance of the divider is R1 || R2, which is 1/(1/R1+1/R2) = 1/(1/1.86+1/1) = 0.65*R2. Therefore R2 = Zout/0.65.        

The AD725 has high input impedance so the exact output impedance of the divider is not important, it just needs to be low enough for the DC restore clamps to work properly. If you choose the recommended 75&ohm; then R2 is 75/0.65 = 115&ohm; and R1 is 115*1.86 = 214&ohm;. The closest E24 values (5% tolerance) are 120&ohm; and 220&ohm;. 



In your related question the composite images do appear to be overdriven, so reducing the levels may help.