Judging from my tests of mapmaking, the #poptype command only affects the type of recruitable units available at a province, but not its initial independent defenders...
Thus it is difficult to strengthen the defense of independent provinces independently from the independent setting,
especially not in a random way, i.e. I wish that there would be a map-command to say that the random independent force is to be set at global independent strength +/- something.
EDIT-PS: Of course, while setting the poptype one can set defenders manually as well, but randomness would help to allow oneself to enjoy one's own maps.