For pad 7-8 and 12-13, since the solder mask shape is not a simple offset of the copper shape, draw the copper pad with no solder mask (uncheck F.Mask in pad properties) and then draw a custom shape on the mask layer.
I would not do this, but use an “aperture pad” instead. An aperture pad in KiCad, is a pad without copper or pin number, but you can use it to make custom sized apertures in other mask layers such as solder mask and solder stencil. With an aperture pad it is also easy to make rounded corners for the solder stencil, and this improves paste release, and thus a more uniform amount of paste on the pad.
It may be a problem, because there is always a placement tolerance between different layers, and when you are working with features that are 0.25mm small, then avoiding stackup of tolerances is always something to try to accomplish. If all pads of a footprint are defined in the same way, then the part will self align during the soldering process. If you use these different types of pads, yo at least have to look up the tolerances that your PCB manufacturer uses between those layers, and that is also likely to be different for 2-layer and multi- layer PCB’s. It’s just an extra complication, and I see no valid reason for wanting to add that complication.
I dunno. What do you want the solder stencil to look like? Do you want it modeled after the aperture pads, or like the copper pads?