Considering the failure that is scale in Freelancer, and the lack of lore explaining, we have absolutely no idea how big biodomes are.
Also taking into account the lore concerning how Synth Paste is able to be made in large amounts at high speeds for little cost, there is every possibility that each acre could feed 10 people, maybe even fifteen.
Without any lore or accurate scale, we have no idea how much those biodomes can truly produce. What we do have is lore that the Freeport in Theta sustains a large amount of the Corsair food demand. This is demand left over after the (fairly poor) crops the Corsair grow, and the (fairly large) amount of goods and food that they pirate.
The Outcasts have a far smaller demand for food than the Corsairs, this is made evident by the lack of any lore saying they have massive starvation to deal with.. That lore is tied only to the Corsairs. As such, we can safely assume the Outcasts grow and pirate almost all of their food products, relying relatively lightly on the Zoners. However, even if they do rely on food from the Zoners, they have Freeport 10, which is next door, to supply most of it.
Having half the number of biodomes to Freeport 9, Freeport 10 can certainly produce half the amount of food to Freeport 9. Allowing the food at Freeport 9 to go to the Corsairs, and the food at Freeport 10 to go to the Outcasts. That said, there is no lore, that I can find, saying the Outcasts rely on the Zoners at all for food products, and instead use them as a useful group to peddle stolen goods to, or buy goods they can't acquire through piracy or their own production.
So long as we're okay with using other sci-fi stories to act as precedence, I'd like to call into consideration EVE.
The stations of which can be as large as 70 cubic kilometers (70km length, width, and height), which is 70,000,000,000 cubic meters. EVE is well known for using proper scale. Lets assume Freeport 9 is a tenth that size, including the biodomes. 7 kilometers in each direction.. 70,000,000,000 * 0.1 = 7,000,000,000. 4 Biodomes account for roughly 75% of the station's size. 7,000,000,000 * .75 = 5,250,000,000 cubic meters. Lets say half of this is available for the actual growth of the food. 2,625,000,000 cubic meters. Lets assume that 25 cubic meters can sustain a single person for a year. It'd probably take only half that. 2,625,000,000 / 25 = 105,000,000 (56 million). That's 105% of the 100 million we're going with. Assuming the Corsairs' population is more around 300mil (Malta is at 215mil), and that 25 cubic meters, with Synth Paste being produced, can sustain 2 people, that's still 70% of the population being fed by Freeport 9.
With "Synth Paste is very adaptable and can be grown in huge quantities" (source: wiki) I'd actually put the number of people being sustained on 25 cubic meters to 5, instead of 2 people..