I personally would never go near a german phalanx whatsoever, even if they are engaged by your infantry. I had a bad experience once.

I was clearing up a routing german phalanx with a unit of light chariots. Somehow, the unit recovered its morale as I was clearing them up. The German phalanx didn't even need to recovered its formation, before it routed my light chariots. I wouldn't touch a unit of german phalanx with a forty-foot pole, when using light chariots.