It's quite possible that it would be a balance issue, but here is how I see it. As it stands, Fertile just gives more bonuses to non-synthetic races than organic ones. If it would be a problem for the ability to treat them in similar ways, that strikes me as a more fundamental balance problem with synthetic and non-synthetic, rather than something that has to do with Fertile.
And yes, you are correct that Fertile is a Mimot ability, and can be chosen for custom races (which I play almost exclusively).
Personally, I think that the Tin Cans are at least somewhat balanced out by the fact that growing population locks out their construction cue from other build options. Plus, worst case scenario, they can't get more than one pop per turn due to lack of production overflow in the cue.
