That sounds cool, though with all of Robins projectiles being so horizontal and Mac not having a very good jump do you not find yourself damaging him?My issue with the Sheik/D3 team is that D3 has most of the kill moves, so he would benefit from an aggressive play style. I think it'll take some time before we find our roles..
As long as we play carefully, it's not too hard to avoid hitting each other. If I recall correctly, Mac can basically dash underneath Thunder and Elthunder (and maybe Thoron). Since we play customs, I use Fire Wall to keep my enemies off me.
Also, the thing about Dedede is that because his moves have such high knockback but he's slow, he has a hard time applying solid pressure constantly, which Sheik excels at (while suffering from a relative lack of kill moves). Consequently, if you can use Sheik to wrack someone's damage quickly into D3's kill thresholds, then toss your enemies into a smash attack or something, you can use each character to cover the other's weakness.