I believe its down to the EO what types of machines class as walkers and what the rules are even if they are using the FRA ruleset. But before we even reach what weight advantages they might get you have to define what a walker is.
I know that for the feathers the legs must have 3 axis of freedom, so that puts cam-based systems like Anarchy out. However I remember chatting with James and Grant and even their 6 legged spider, who's legs can move in all 3 axis, would not count as its still effectively cam based at its core.
Even going with 'in the spirit' of walking doesn't work as then its ambiguous and people may... what am I saying... they WILL disagree over what is legal and what isn't.
As far as I can see the only way to do it and be totally legal is to build a Hexapod or Octapod with servos, linac's or hydraulics, then you have total control over each leg individually.