Sorry I'm late to the party. Thought I'd write up a bit on Mega Man's doubles stuff.
I've been playing team battle quite a bit lately as Mega Man (w/Greninja and Duck Hunt as teammates). In my experience, he's an amazing doubles partner. Of course I can hold my own when it separates to 1v1's, but he shines in team play. Dash attack can capture both opponents while allowing your teammate to get in position for a follow up, lemons can give your teammate an opening to start/land a combo while the opponent is in hitstun, and MB can be used at diagonal angles to cover different options as well as being a piercing projectile. He's moderately heavy with a good recovery, so he works well as a stock tank, and has great aerials that combo well when your teammate sends an opponent flying your way. Also, a charged fsmash can be devastating to unleash on someone who is trying to fight your teammate. Once I can SH lemon consistently, Duck Hunt can/pidgeon + lemon camping is going to be really funny.
Some team specific things, crash bomb activated Greninja substitute is good, although gimmicky, as the substitute attack is really hard to punish. Also, due to Greninja's speed, he can pass the CB quite well. Deploying Rush to save your teammate is really good if you're in position for it. I don't know if it gives the teammate back their double jump, but I don't think it does. I'd say it's better than Tornado Hold for that purpose, as it does 0 damage and they're free to air dodge or whatever straight off the boost. It requires more timing, but it's better.
G&W is an OP teammate because of the bucket. Other than that, I don't know other team combos with Mega Man. He teammate strengths are probably stage control, team % combos, edge guarding, and killing. In my DH/Greninja & Mega Man team battles I get around twice as many kills as both DH or Greninja and that's by design (and my teammate is no slouch). Mega Man does get pushed around when the other team doubleteams him, which can hurt his stock tank ability, but your teammate can bail you out of those situations and help you regain initiative or at least reset to neutral if you choose to Rush out of your disadvantaged situation.