So the premise is rather simple: food can be moved from where it is grown. Worlds would have Agricultural policy settings, and would be something like:
1. Subsistence agriculture (grows only what is needed on-site)
2. Export Agriculture (allows production of surplus agricultural products, can be exported on freighters)
3. Import Agriculture (allows extra foodstuffs to be imported, which boosts population growth and population cap)
This could benefit from more options, such as allocating more population to Agriculture, like 10%, 20%, 50% of surplus or something like that.
Waterworlds could be used for Fishing (perhaps after "seeding" them with life)
Worlds with a colony cost above 0 would suffer significant reductions in Agriculture efficiency (due to not being able to just plow a field and plant corn wherever you want)
Agriculture would have a "race" of crops, which all come from a specific world. There would be a new "race" of crops generated alongside every species. The agriculture Efficiency for a particular "race" and planet would be dependant on a few things.
1. Terrain similarity. Crops from a Jungle world won't do well on a Tundra one. It may be possible to grow some crops in the wrong biome, but that would depend on the (possibly imaginary, but maybe not) areas of varying climate.
2. Terrain Fertility. This would be mostly seperate from crop race. Something along the lines of "Jungle +30%, Tundra -65%, Chaparral +10%, Desert -20%"
3. The amount of habitable area on the planet.
Some species may be able to eat another species' Racial Crops, though at a penalty if that. Perhaps there could be a Bio/Gen research to boost efficiency of a particular crop race (you could consider it selecting ones that are edible to your race, and farming those instead of the others)
Crops might be able to be measured in tons, though that might be a problem for transport
Maybe Infrastructure-reliant colonies could be set to Import, which would free up infrastructure for Population.
Lastly, this system in particular might benefit from allowing a planet to have multiple terrain types, on a percentage scale. For example: 60% Jungle, 20% Temperate Forest, 10% Desert, 5% Tundra, 5% Urban.