These are all great things to discuss.
Odd interactions with specific characters are a good place to start. I have seen Ivy get healed from Dins. Obviously G&W, Ness, and Lucas can double-absorb Dins as well, which is rather potent. Snake's traps are set off by Dins, so I think Snake is unable to use dsmash if Zelda is ever going to use Dins (damn drive-bys). I've been partnering with Snake lately and its fine, we just have to adapt.
Honestly, I don't care much about who my teammate is. I almost entered doubles with a lvl 9 Zelda CPU the other day. I love the teamwork, cooperation, and coordination that goes into teams matches, but I can play teams with pretty much anyone (don't even care about skill level...like 9). The extra bodies limit my opponents' options in ways that allow me to read and outspace them. Hell, I even enjoy 1v2ing more than singles.
I think you meant Skyloft's slopes screw up Ivysaur, but I don't think discussing which teammates screw up stages is really appropriate here. As for Zelda's good/bad stages, I would recommend not going to Norfair. It is a large open stage, but you can't cross it quickly to save a teammate or 2v1 because the platforms or too high, or they're moving, or the stage is just too damn big. I feel similarly about Dracula's, but I don't have enough experience to confirm. And since nobody else has experience there either, I do fine for now.
I like to talk with my teammate. Compliment them when they do well. If your plan doesn't need to be secret (the opponents can't stop it anyway) then just say it. Personally, I love when my teammate and I have a casual conversation while we play. We're usually good friends, so its easy to have a conversation, and it usually makes our opponents foolishly aggressive (trying to take advantage of our "distraction") or ludicrously incompetent (as they get salty because we "aren't taking them seriously"). Casual conversation with my teammate turns into casual conversation with the audience, which turns into casual conversation with the opponents, which turns into a blowout. Anytime you get a team combo, acknowledge it out loud. This gives your teammate confidence and your opponents sodium.
Oh, and remember to thank the goddess Din anytime your teammate combos, gets out of a combo, or gets saved by Dins Fire. Or as my scene is starting to call it, Din's Bull****. Praise the lady.