If so, is ROB or Falco a good choice? They are secondaries.
I can't speak for anything on the Diddy vs Snake matchup, but the ROB vs Snake matchup should be significantly easier for ROB. Since you can laser his nades you force him to approach as he can't nade-camp you, and once he's off the stage ROB's recovery lets you chase him regardless of how high he chooses to recover (only a few characters, including pika and MK if you can play them, can do this, which makes it extremely annoying for a snake to recover). A well placed gyro can stop his mortar slide (well, good positioning in general does that, too). Your aerials will own anything he has in the air.
But the most important thing, regardless of which character you choose, to fighting snake is knowing the range on his tilts. If he's walking towards you, get the hell out of the way. He's going to ftilt/utilt you, and you're going to die. Go into training and get used to how far his utilt/ftilt reaches in front of him, and stay outside that area unless you're trying to bait him.
Grab his cypher if he chooses to recover close to the edge, and he loses. Grab his body if he chooses to recover close to the edge, but don't hit him, and he has to c4 jump his way back. Since Snake is so focused on his ground game (with the occasional autocancelled aerial), you need to focus on nana's. Snake's strengths are in his insanely powerful tilts and his ability to control the stage. Diddy's strengths are in his ability to combo and create opportunities with nana's.
tl:dr version - Learn the range on Snake's tilts, and don't be in there.