If I was you, I would get the adjustable boxes and do it that way. I put some of them in for a friend who was going to put a workbench on the wall below the boxes and put up some pegboard or other material on the wall. All you have to do is leave enough room around the boxes with the finish material so they will adjust out.

Better plan on a large box and large faceplate. For items with different voltages, the wall box must have partitions to keep things separated. Masonry boxes are probably not what is needed for your application, but this video explains the concept of partitions. Here is a link that might be useful: Box

"There is always a hot wire going to the switch and in the switch box." Gee - you may want that to 'always' be the case - but it is definitely NOT always the case. What is ALWAYS the case - is that a regular light switch (not a three way, four way, etc.) is always in Series with the light. Yes, it would be good, to have the HOT controled by the switch... but for example, years ago - I got zapped by a light - because the Hot was gong to the lamp, neutral to the switch... and I was an idiot and hadn't killed the power at the breaker... Anyway - in a bathroom which has its only outlet on the lamp - it seems a good bet to me - that we are probably dealing with knob & tube wiring... I have seen some wierd electrical in my days... so I try not to assume. But adding a GFCI outlet; is definitely something the average person can do - after some careful reading.

The code requires 20A for good reason. It also bars other outlets on the small appliance circuits. We can not advise you that violating the code is a good idea. Plugging in things like electric tea pots into a 15A circuit that is shared with countless other things is problematic no matter how it is protected, if not now when you know that you've done a butcher job on the wiring but to some unsuspecting future occupant of your kitchen.