(A coordinating conjunction is a word that joins two elements of equal grammatical rank and syntactic importance.) (website definition)
Subordinating conjunction used as a subordinate conjunction: (website named exception 'so'.)
Grace is saving money so (she can buy her own horse.)- dependent
Question: Can the others And, but, for, nor, or, yet work in the same capacity?
You can put most of these coordinating conjunctions in front of ''she can buy her own horse'' and re-work the sentence.
Grace is short of money but she can still buy her own horse.
Grace is short of money yet she can still buy her own horse.
Grace is short of money and ,for now, can still buy her own horse.
So they are not joining elements of 'equal' grammatical rank.