Am I correct in assuming that "estar" goes with an adverb, while "ser" goes with an adjective?
In general, "ser" is used with permanent or non-changing characteristics, while "estar" is used with temporary or changeable characteristics.
You can use either one with an adjective or adverb, it just depends on what the particular adjective/adverb is.
So, for instance, "I am tall" - adjective but permanent characteristic - Eu sou alto. "I am happy" - adjective but changeable characteristic - "Eu estou feliz".
Could I say, 'Eu sou bem' or is this one of those phrases that while technically correct, in general usage it isn't used?
Eu sou bom= I am nice(assuming youre a good person) Eu estou bem= I am fine(assuming youre healthy)