issue @ 18:25

Hi Abhi,
It sounds like you accidentally changed the rotation orders on one of the foot controls.

There are different rotation orders because it will tell Blender the order in which to calculate them.
XYZ, YXZ, etc etc.
Roughly speaking  if you have the same inputs but different rotation orders - you are going to end up with a different orientations.
(Think of the orders 1. walk forward 2. turn left. And now reverse those instructions.....Turn left, walk forward.  They are different)

Why can't you copy and paste the keyframes from Euler to Quaternion?
Blender matches the animation channels by name, and the euler rotations are "X Euler Rotation", "Y Euler Rotation" etc.
But the Quaternion channels are called "X Quaternion Rotation", Y...., Z.... and "W Quaternion Rotation" (yes there is a 4th channel)

What's the Difference between Euler and Quaternion?

Euler rotations are named after Leonhard Euler (pronounced "Oiler"), they are much easier to work with for animation, however, what ever rotation order you have they are prone to Gimbal lock when the middle axis hits 90 degrees.  This is a pain in the butt when animating.

Quaternion rotations don't suffer from gimbal lock, but they do have a few notable drawbacks.  They always rotate the sorest distance between 2 orientations.
So you can't rotate 181 degrees without adding a keyframe in the middle (or flipping the quaternion - what the what??).
Plus you have 4 animation channels to deal with which don't behave in a very intuitive way in the graph editor.  Which makes polishing any rotation curves REALLY difficult.

For this reason most animators stick with Euler rotations and just deal with gimbal lock when it occurs.

So was your solution a good one?

Kind of, it was probably better to stick with both of the Feet in XYZ Euler, (rather than both in Quaternion)

But if they are both the same, then copy and paste will work as expected, and lots of animators do use Quaternion on the controls that move in world space. 

Anyway, hope that helps.