Hi! When I parent the right leg IK of my model to the spine, everything works as expected. But when I parent the left leg IK to the spine, it completely messes up the rig when I go into Pose Mode: it not only rotates the entire skeleton, but when I move any of the bones, the model starts spinning around. Any idea on how to fix this?
Most likely your IK chain length will be set to 0 (this is the default setting) you probably want it to be 2 (or however many bones are in your chain)
When it is at zero it will run all the way up from child to parent until it can't go any further (which is probably your root bone)
So it screwing everything up.
Also I can see you have 3 broken drivers in your driver editor there. You might want to look at why they are broken and then update after fixing it. (that's the red lines under your drivers)