adding arm tags to profile makes it break

That sounds strange indeed! I don't know the answer, but some tools to help debug it.

First, my understanding is it is best to make the Frontal profile the first sibling. I thought the Walk behavior assumes the first child is the non-walk profile (it does not look for "Frontal"). So I think some problems can occur if the standing profile is not first.

Next, in the Properties panel, have a look down the bottom of the Walk behavior. Expand the "Views" and "Handles" sections. In Views you should see two - Left Profile and Right Profile. (Remember standing should be first - it will no appear here.) You can also peek into Handles which will show all the wrist, leg, etc handles needed by Walk for the profiles it found. (Not needed for the standing profile - just Left and Right.) I would see if anything looks strange in here, and see what changes when you add the extra arm tags. It might give you a clue to the tagging problems you are having. I suspect unexpected things are being bound to.