Check out the https://www.galciv.wikia.com/wiki/Population article. There's a link to the GalCiv2 Wiki in the "Databanks" section. Just clik on "Databanks" button at the top left of the screen right under the "Home" button.
The 6.0B max pop limit is your "food limit" based on the food capacity of the initial colony building. The initial colony building will support 6B people. The 2.5B limit is actually the planet growth limit beyond which the planet will no longer grow. You could if you wanted to bus in population from other planets using a transport or colony ship. You can do this just by putting a transport/colony ship in orbit and then when you launch it and you get asked how many colonists/troops to load just say 1 (I guess this is the ships minimum crew.
However it's usually not real practical to bus pop around like this and so the "real" max pop is the lower of the amount of pop that you have the food to support and the maximum the planet will grow basd on it's PQ.
The following is excerpt from the above referenced article.
Here are numbers for Maximum Population per Planet quality in billions:
PQ 1 = 0.16 b.
PQ 2 = 0.54 b.
PQ 3 = 1.28 b.
PQ 4 = 2.50 b.
PQ 5 = 4.32 b.
PQ 6 = 6.86 b.
PQ 7 = 10.24 b.
PQ 8 = 14.58 b.
PQ 9 = 20.00 b.
PQ 10 = 26.62 b.
PQ 11 = 34.56 b.
PQ 12 = 43.94 b.
PQ 13 = 54.88 b.
PQ 14 = 67.50 b.
PQ 15 = 81.92 b.
PQ 16 = 98.26 b.
PQ 17+ = 100 b.
Notes on maximum growth-achievable population
Yes, a player can still reach planet's food limit by transporting people from other worlds. This would make sense in case a player tries to maximize tax revenue, or wants to make planet less sensitive to enemy invasion.
If you transfer to a planet more population than a planet can feed, all excess population will die off when game generates next turn.
So note that it's not worth it to put a farm on planets PQ7 and below, nor more than one farm on a PQ8 but PQ9 and could support 20B dependent on your approval ability. 20B is for all intents and purposes that max "reasonable" population.