Allow me to clarify.
On a strict rationing diet, The diasporex manage well, but with any sort of agriculture, there is a general loss of resources over time, even in a near-perfect system. This is why phosphorous mines are so wildly profitable.
Even if the Earthseed is huge, there is only so much resources they can devote to livestock. because of this, protein tends to be a rare treat. Even High Senators rarely see it, and may be lucky to see fresh meat once a year instead of Soy, or other synthesized proteins. This is just the horrible truth about living off world: space is at a premium.
This means whenever a supply convoy is found, they are raided and anything useful is taken. Things like raw materials, foods, fuel cores are taken, along with a few ships to be broken down for raw materials. Things like weapons of war, farming supplies, etc. are ignored.
Another thing limiting the agriculture is that the Earthseed has some of the very last examples of pure Terran biomes, and replacing them with agriculture would destroy them forever, along with eradicating any surviving species that live in said biomes. These are held to be some of the Earthseeds greatest treasures, and any foolish Senator who suggests it quickly finds himself disgraced and voted out of office.
The Goulds tend to be used by the other races as their "home" base, since many still prefer their own company to that of humans, and some cannot survive in an oxygen rich environment. While there are several of them, most of them are provided to the partners of the Diasporex and are far closer to floating cities then colonizing vessels at this point.
They are not modified, its closer to selective breeding. When a couple desire to have children, their DNA is screened for the likelihood of producing a psyker for a child. If it is a high probability, they are barred from breeding and suggested to adopt. The High Senate recognizes that this is a rather draconian approach, but it's a better alternative to having an Alpha Psyker lose his mind and turn the Earthseed into a lawnchair (yes they can do this.)
When one is born, usually in defiance of the law (where the parents are severely punished for risking the safety of the fleet) or by sheer chance, the child is taken by the Naval command and raised in highly controlled environments so they can grow up to manage their powers responsibly. They are NOT above killing the psyker if they prove to be unstable.
The end result is that the Psyker gene is weaker in the Terrans then it is in Humans, simply by force of breeding and social pressures. When your home is a space ship, it's just too dangerous. Also, remember that a single Oddboy can be put on a stick and used as a 15cm lance. Imagine that coming from INSIDE the ship where there is no armor plating!
