Clarification on solder mask aperture in PCB?
You have to check this with your manufacturer. They know their process, only they can tell you what is and isn't possible. 

I've seen many different values. I have also, on a number of occasions, found that the PCB house wants you to have 0 solder mask expansion, and their tools will provide the correct solder mask expansion for you, so they can ensure that you are not missing solder mask where it is needed. Contact your manufacturer. 

You can also not just decide to have less than needed solder mask. As you said, there can be misalignment of the mask, and if you don't leave enough space between the mask edge and the pad, this can result in your part not properly soldering, as there is mask in the way.