I have a similar sized tank. Too small for a blue tang. I would scratch the mandarins unless you're pretty adept at feeding live food to them. I would also add the firefish first. I have a pair of ocellaris, komini tang, flame angel, one spot fox, six line, blue damsel, banggai, and a purple pseudochromis. It's about maxxed out and everyone is moderately aggressive so it balances out.