The solution is that you get ships on a list, like with the Classic nebulas. 1st human faction you bring under your nets you a flagship, the next gets you a specific kind of turret, etc. That way it doesn't matter who you get, only how many.
Mix in the "pay for their rebuilding" schemes Chris mentioned and you can replace "number of factions you save" with "strength of human presence" as the determining factor for the rewards.