Can the brat do the Walking around. Faster gait, shorter steps if it has 422 servos or do you have to use 485s or 645s?
there is a few questions here.
the brat can do the walking around yes.
a faster gait running shorter gait steps would be ok and 422’s would work fine. if you really wanted to beef it up you could use 485’s but it would be over kill on a standard brat assembly. 
not sure how much faster you will get it due to the balance. you could sustain a faster gait but not sure how fast you are talking.