You’re asking about how frozen are some multiword expressions.

Pomme de terre is a lexeme in itself, so much more frozen than the others that it might not even be in the same category.
Mauvaise herbe and grand magasin are completely frozen, as far as I can tell (figement complet).

I found some theory here¹ (page 9ff.), that answers your second question.

Basically, if you can fiddle a bit with the expression, inserting words, changing some for synonyms, etc., it’s only partially frozen.

¹ masters thesis of Corinne Lédée (2011)