Like the Band of Homo Sapiens gives a free Tribal Guardian when you found the city, the Colonist and Pioneer can give a free (or a few) Caravan to boost production early. It could go a step further: Like the Tresure Chest can instand build the Museum we could design a Colonist / Pioneer Supporter Unit that is immobile and can autobuild certain buildings instantly: Bakery, Aqueduct... When the conditions are met. I think a unit can't build multiple buildings before consumed, so we need to make the pionoeer/colonist give more then 1 Supporter Unit. If you don't use them, they will disappear after 1 or 2 turns. At the same time the Colonist can give 2 free Immigrants and the Pioneer 5. These Immigrants should also be immobile and disappear if not used after a turn.
This would avoid free Buildings which conditions aren't met and also avoids the bug for the AI.