At present, there were only four imperial marshals who enjoyed these privileges, though tonight they would become five when Count Reinhard von Lohengramm was added to their number. Moreover, it was going around that Count von Lohengramm would also be made vice commander in chief of the Imperial Space Armada, placing half of its eighteen fleets under his command.

Hi, the problem in the above is not WILL vs WOULD (which, by the way, is the correct option here, to indicate a future event that may occur under specific conditions—here the condition of adding) but rather WAS ADDED, which should in fact be WILL BE ADDED (here WILL is correct as, from context, it seems to be a certainty). There is no reason to have the past then here, for the same reason that WOULD ALSO BE MADE was used in the very next sentence. There, WOULD is correct as it denotes a possibility, not a certainty—see "it was going around."

For WAS ADDED to be correct, lots of changes need to be done, as below:

Until then, there were only four imperial marshals who enjoyed these privileges, though that night they became five when Count Reinhard von Lohengramm was added to their number.