Fastfall Fair -> Side B is the best way to momentum cancel from what I'm seeing here.
Airdodging isn't as good as fastfalling an aerial because fastfalling actually stops your momentum a bit, and you can do it during an aerial. Whereas you can't fastfall with an airdodge, so even if the airdodge ends earlier, the frames that you're fastfalling during your fair actually ends up helping you more even if you need to wait a bit longer to use the move.
This would be wrong if you had a braking move like GaW or Pikachu, becuase in that case you always want to do what ends the fastest so you can totally stop your momentum ASAP. But ROB doesn't have a braking move (on that note, ROB's jump actually doesn't affect his momentum at all so jumping after an aerial is pointless with him) so Fastfall fair -> Side B should be the best if Side B really does slow momentum like you guys are saying. And it should, now that I think about it.
And logically, tapping B during the Side B should help you out. The example someone gave earlier about how you keep your momentum with Fox's Shine or Side B is flawed because you're assuming that the moves totally brake your momentum like a GaW bucket does. That's not true; it just speeds up your momentum so that you kind of..."teleport" to where you would've ended up anyway, but faster. This means it's the same end result and there's no reason to do it. With ROBs Side B, by fastfalling an aerial you're already starting to regain control in the air the second you do it. After the Fair ends and you begin your Side B, every frame that's going on, you're continuously "fighting" the momentum and getting control back more and more every frame. It's not a "teleport" move like Fox's specials are.