You should be fine. However, the bose cubes are meant to be used with their acousticrap module that has the crossovers. You won't be able to do it properly without that, however, I figure the worst you could do is blow them which in any case is extremely unlikely to happen as rear backs as there is very unlikely to be enough low bass in those channels to really damage them.
I would suggest you try using the system in 5.1 and 7.1 and see which you prefer, they may be fine just to fill in to the rear, or you may prefer just to stick with the paradigms in a 5.1 array. But experiment to see.