How to politely reject a professor's request for work that I was recommended for?
"While normally as a graduate I should be very thankful and jump on this, I simply cannot do the work asked due to time and I believe there would be a more suitable candidate for the work."

That is more or less exactly what you should say. Express your gratitude to Professor A. If you have a major academic obligation taking up your time, like comprehensive exams or something, mention that. Do NOT CC the candidates you have in mind, but do mention to Professor B that you can recommend someone else who you think would be a good fit.