I think this answer should more or less answer your questions, although it focuses mostly on the provincial tax-collection of the late Republic, which is what I know most about. During the Principate collection of the vectigals and of tribute mostly moved away from the publicans, who would eventually be pretty much totally replaced by the time of the Antonines.

As for paying soldiers in salt, I think this should help you. The myth that Roman soldiers were actually paid in salt seems to come from a misunderstanding about the meaning of the word salarium. Salarium means something like "salt-stuff," and doesn't mean "salt as money" but instead "money for salt," that is the money that a soldier or magistrate was paid to make up for the expenses he ran up while outside the city (on equipment, transport, food--salarium can also mean a meal).