Hi Anna
I'm in Europe and so always measure tempo as Bars-per-minute. Jonathan's point is very valid, so it's all down to convention. I believe that in the US they still measure in feet and inches!!
As for dancing fox-trot to waltz music - you just have to dance the slows a little quicker (as, for example in a feather step). Dancing waltz to fox-trot music is equally simple - in fact i often do this at social dances where ther's not enough space for a real fox-trot. Instead i practice slow waltz natural and reverse turns with the normal '1' occupying two beats of the foxtrot music. I'm sure you're aware of all this.
On a good night i just do the 3 steps to the 4 beats.