We don't currently have any special Mopex/Apex namelists for crowded fields. Many people have used the default namelists. If using the GUI, see Cookbook Recipe 7, http://irsa.ipac.caltech.edu/data/SPITZER/docs/dataanalysistools/cookbook/7/. And the command-line namelists are identical.
There are 2 things to consider. One is whether the Mosaic task has done a good job given the low coverage. Check whether there are uncorrected radhits on the mosaic. The module "box_outlier" is on by default and designed to work with coverage as low as 3. If needed, it could be made more aggressive by lowering the "BOX*THRESHOLD" parameters in the MosaicRmask block.
Second is whether Detect is optimized for your mosaic. The default namelists should be okay for the bulk of the field. You might do a
second special run for a crowded area -- the main parameter in Detect is Detection_Threshold which you could raise for the crowded area.
You could also raise Min_Coverage if radhits are slipping in.
But there will always be some fields that are just too difficult for an automated program to do properly.