Actually Budwig herself warned against using other supplements with the protocol. But to my knowledge she didn't clarify which class of supplements would have a negative effect which is too bad.
For example I can't imagine supplementing with Iodine would be a bad thing since we know our diet is deficient in it. And there are some Budwig offshoots that preach the Budwig protocol except with a broad range supplements added. Some extra clarification would have been helpful - unless I was missing something.