Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.
That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.
The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.
Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.
Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.
Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.
You are correct. The field of building science is gradually moving away from friendliness toward vapor barriers in exterior walls, and you certainly should not put one in an interior wall or floor. Not a great idea.
In addition, it would make more sense to use batt or spray foam insulation between the floor joists as opposed to cutting rigid foam to fit in those irregular spaces, which would be a nightmare. Finally, most foam insulations are themselves vapor barriers, so if you use foam, you will be creating much of a vapor barrier anyway. For this reason, ideally you would use mineral wool batts (sold under the name "Roxul" in the USA) in the floor joist bays, as they are highly-insulating and vapor-permeable. As an added bonus, they block sound very well too--useful for insulation between floors of a house. I am re-insulating my house with this stuff and just love it. Unlike fiberglass, mineral wool batts are not moisture sensitive, easy to work with, and highly effective.
Best Answer
This would probably work, but "better" would be closed-cell spray foam or rigid XPS foam boards. Both of these will also act as a vapor barrier and have a higher R value per inch of material than fiberglass or rock wool. They are however more expensive, and spray-foam is not typically something that is DIY-installed.