Столь (or настолько) means quantitative "so" (meaning "so much"), and can be replaced with так or такой anywhere it cannot be confused with qualitative or classifying "so" (meaning "in the same way").
This means you should avoid using так when quantitative meaning is not the only possible one.
Compare two sentences:
Я не настолько накрашена, чтобы принимать гостей.
Я не так накрашена, чтобы принимать гостей.
The first sentence unequivocally means "I'm not wearing enough makeup to welcome my guests".
The second one is dubious: it might mean both "I'm not wearing the right makeup" or "I'm not wearing enough makeup". Though this sentence is a little bit ungrammatical if parsed according to the first meaning, this first meaning is what most people would perceive from this sentence.
Answering you second question, yes, столь and настолько are complete synonyms, столь being the grand style form.