We still manufacturer magnetic thin films on flexible media, for the last few 3.5 inch floppies and other purposes, and I'd imagine that you could get away with putting a very low resolution magnetic pattern on film capable of a much finer one (though not the reverse)
Not necessarily--not all magnetic thin films are the same. Ones capable of storing a higher density of magnetic patterns have a higher coercivity (i.e., it takes a higher magnetic field strength to change the magnetization). The write heads in drives designed to write on lower coercivity media aren't strong enough to write on the high coercivity media. Which is why you can't use a 5.25" HD (1.2MB) floppy in a DD (360KB) drive.