I completed minimac's challenge with
22 builders, two of which were wasted selecting lemmings facing the wrong way when releasing the crowd. If you discount those, I have solved the level with 20 builders and no other skills (lose 3).
As for "fewest builders while using other skills as well", here are my current (glitch-free) records:
* Tricky 13, 5 builders and lose 1, or 6 builders and 100%
* Mayhem 25, 5 builders and lose 2
The Mayhem 25 five-builder solution is a thing of pure elegance
It uses every single non-builder skill you're given.