Well, Corsairs get their food currently mostly from Samura or their puppets, such as FA or the Hogosha. Junkers and unaffiliated Corsair smugglers of course don't miss the chance to earn some artifacts on Crete. Those're the most often ones. Gaians, freelancers, Cryer, IMG, Liberty Rogues are among the less likely guys.
As for Zoners, they aren't interested in supplying someone with purpose, that's what I think.
As for Crete soil, I think Crete surface had Greece in mind, thus lack of fertile land. To fix it up, Corsairs need to run some terraforming proccesses, which they can do (if Sylpheed's Rogues stole the best engineers in the Sirius, why can't Sairs take a couple of scientists from Camridge?), but can't, as it'll change them too significantly, especially for our devs. Current relations with Kusari are another choice, as well as agressive attempt to retake Camridge or Stuttgart.
Relationship with Synth Foods would be one more interesting option, however current Synth Foods is dead, unfortunately.
Corsairs aren't numerable - at least I haven't seen any rumour suggesting that, and if people are hungry all the time, I think they're not going to make children like rabbits - especially if they're unable to feed them.