Heres my question, lets say a faction has 1 city on this planet and i was to build on it and make an agreement with them to do so. Once I start building and the planet gets a population doesnt that population and planet income help the faction?
It has the potential to help, but it also has the potential to hurt, depending on how the planet stats end up. Tax income does take overall civilization level into account, but also factors such as Morale, which is dependant on the specific facilities being built (see http://rules.swcombine.com/?Planetary_Economy for more information).
Also worth noting, in case you aren't clear on the issue, that only government factions benefit from taxes (planetary income, which was your phrasing). Other factions can only receive facility income, which has a similar problem with others building on a planet in that it also depends on planetary stats, but the faction will not profit from it at all unless the facilities are owned by or assigned to the faction.
Edited By: Syn on Year 10 Day 3 15:46 ____________