The front knuckles are different, but I think they will work fine. And I'm sure you've noticed that the Si has rear disk brakes whereas the dx has drums.. unless you have an 88si with the drums, if that is the case, you can't use that rear supension on a 91 dx. It is totally different.
If its an 89-91 Si you're in the clear all the way around. The disks are a nice upgrade. Be sure to also change out the proportioning valve, or its going to stop weird.
Is there something wrong with the front suspension on the dx?