One important thing about Dedede's edgeguarding, in my opinion, is how fast he fast-falls. So, what you can do relatively often is full jump off the stage, so that covers if they recover high and maybe makes them try to avoid it, so then they recover mid or low. If they then go mid or low you can either fast fall into an aerial or just use down-air, which pops them up into a perfect position to forward-air. You fall so fast that you can usually get down to their level very quickly. If they are in front of you: Fair, if they are below you: Dair to Fair.
A couple of times when you were edge-hogging Fox and you repeatedly went for back-airs, you could have thrown in a forward-air to finish him off on the back-airs that didn't send him too far. One thing I've been practicing lately is that: quick turn-around forward-airs. That is, either falling from the ledge and turning to use Fair or doing a Bair into a turnaround Fair. I'm working on the timing because obviously if you attack too fast after jumping you won't turn around.
You basically want to force Fox and Falco to use their up-B's instead of side-B's because there is that period of time where they are stuck in place, and you should be up in their faces with a Fair at that point.