That is a really cool design.
Just off the top of my head I would guess that an HS-422 does not have enough torque for your design. To know for sure I would to know the measurements of your robot and the weight distribution.
But here is why I would guess a HS-422 is not strong enough. At 6v an HS-422 has 57 ounce/inch f torque or 4.1 Kg/cm. If 80% of your robot's weight is above those ankle servos, then those ankle servos will have to lift 1.6kg. With a 4.1Kg/cm of torque and 1.6kg of weight the servos will only work with an action arm of 4.1/1.6 = 2.56 cm. Joints are levers and the longer they get the stronger they need to be to carry the weight. Those ankles seem to carry most of the weight of the robot with an action arm of about half the robot. The "about" comes from not knowing the weight distribution of the robot. If most of the weight is in the top cross member you need more torque, if most of the weight is in the feet you need less.
But assuming your robot lifts at the ankles, stands on one foot, and has an even weight distribution above the ankle totaling 1.6kg, it could only 5 cm tall. It looks a lot taller then that, perhaps 20cm tall? So you would need a lot more torque than that. You have to think about how much weight the servo has to carry, and how long the lever is to decide how much torque you need. In your design it looks like your ankle servos have to be the strongest because they carry the weight of almost the whole robot when it is on one leg.
These are very inexact estimates based on looking at your cool design.
