As far as edgeguarding Fox:
1. Butt bump (under 100% ledge recovery) Fox's illusion and firefox to knock them back off the stage. This gives you time to follow-up with a b-air and/or up-b from the edge of the stage
2. Up-b Fox's firefox if they're below the ledge or far enough from the stage that you can easily predict the up-b angle.
3. F-tilt or d-tilt Fox's illusion/firefox
4. D-throw from the edge and follow-up with a b-air and/or up-b. I learned this one from DFU
Some of the things I've learned against Fox players is that they will bait your grabs (and punish missed ones), shine you off stage and c-cancel your b-airs & tilts.
That said, spacing and working your neutral game will (ideally) make Fox players less inclined to just nairplane in whenever they fill like it. For example: f-tilt their nair; ground slap obnoxious dash-dancers (once or twice, followed by a headbutt or dk-punch and they'll get the idea, heh); b-air/n-air --> grab combos; n-air --> d-tilt to create space.
Also, abuse platforms for your combos, especially shorter platforms like on Battlefield and Yoshi's as DK's headbutt hitbox covers most (if not all) of the surface. If you're close to the edge during or at the start of a up-cargo combo, players will DI off the edge so pick a follow-up that's safe (up-b, fading f-air, b-air...). One of the things I see Green Ranger do is take Fox (and Falco) to a top platform before starting an up-throw combo because it makes DI much less effective.
Good luck.