You should avoid mask defined BGA pads, if possible.  Avoiding any mask overlap on the pads is a good idea.  The problem isn't solder slumping over pad edges (it doesn't do that anyway), it's the mask creating a solder to pad joint shape that concentrates stress.  If your intermettalic is always good, and the PCB isn't stressed much, you should be OK, but there's no margin for error.

Use the manufacturer's pad size, the via in the pad will be plugged, you don't really need special considerations in mask or paste for this application.  The mask size will be some compromise considering your PCB fabricators copper and mask location and aperture tolerance and minimum mask web capability that keep mask off the pads.  The assembler should control the paste aperture based on their process.

Pete

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