While Matt prefers the simplified JSON interface to swarming, the description.py/permutations.py interface is the primary way to do swarming currently and gives much more control over the model parameters and what is swarmed over. The permutations.py variables specify a range to try for each parameter and the values that it tries override values in the base description.py so simply leaving things out of the permutations file should achieve your goal of a more constrained swarm.
It’s true that documentation is limited though. You can start with Ron’s video and then modify an existing example that has description.py and permutations.py files.