Renth and UberIce doing friendlies right now:
http://www.twitch.tv/Renth814
I'll post more about ICs in doubles soon.
[EDIT]:
ICs in doubles
ICs are most certainly not bad in doubles, they actually wreck pretty hard. But it requires both the ICs and their partner to play a certain way.
In neutral:
The ICs need to be able to control the ground. They have a lot of options and move fast as all hell. This means that their partner has to play on the platforms a lot more. A lot of characters have trouble doing this.
All of the top 7 (and many of the other viable characters) are able to play this way, however.
In general, in teams, the ICs speed on the ground allows them to be the primary aggressor, while there partner is able to play a more defensive or support roll.
The ICs are also able to put up a defensive wall and allow their partner to act as the aggressor.
General strategy (non-neutral game):
You and your partner want to take the center once you move out of neutral.
The ICs are incredible at the "bully" or walling strategy.
What this means is that you create a defensive wall blocking out your opponents partner from supporting as your teammate wails on the victim via pressuring near the ledge/edgeguarding.
This strategy is also incredibly effective in reverse.
Your partner acts as the wall while you focus on chaingrabbing or wobbling the opponent to death.
So, now that that is broken down, these are the characters that most effectively work as a teammate for the ICs:
Fox: Incredibly versatile, can play all roles in doubles effectively. Controls platforms like a monster.
Falco: pretty much the same as fox, but lasers allow him to wall many characters even more effectively. Lasers on platforms often force your opponents to challenge you on the ground, which does not often work well.
Sheik: probably one of the best characters for walling and controlling platforms via needles. In this team, you will usually have to act as the aggressor however, unless you play an overly defensive team
Falcon: This is one of my favorite teams. Both of you can go deep as hell, falcon often likes to act as the primary aggressor. He is very fast on the platforms and has no projectile to interrupt your synchronization.
Puff: This is also a surprisingly fun and effective team. Puff walls like a monster and that allows you to pressure as hard as you want. the only downside is that you almost always have to be the aggressor.
Ganon: I love teaming with ganon. He's like a giant puff combined with luigi on platforms. the downside is that a misplaced aerial on his part makes a dead nana. An effective team, nonetheless.
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Peach: This team is good if the peach plays correctly. This means she has to float above the ICs level and downsmash only on platforms (a loose downsmash from her separates you and messes just about everything up). Her walling ability is good and she does a good job keeping your opponents separated.
Marth: This team is very odd because its one where marth MUST play on the platforms. Often times, you end up trying to control the same space. And that is no good. But if marth plays quickly on those platforms and effectively acts as a wall, it works.
Pikachu: This is an odd one because you are not the aggressor. Pika goes in and does his job and you act as the wall. Pika is a bad wall for the most part. Give him space, because that dsmash will mess nana up, but keep the opponent out.
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Doc: I, personally, enjoy the doc/ICs team, but it has a lot of issues. Pills interfere with you. A lot. Doc does play well on platforms, and he is a good wall, but he's bad at being the aggressor and gets in your way a lot.
Samus: I've had success with this team before, samus basically just needs to shoot missiles over your head and let you do your thing. Samus is a great wall, but she can't play aggressive for ****. Her downsmash is scary for your nana too.
Luigi: Don't even consider using this team seriously. You both serve the same purpose in teams and control the same space. It is not effective at all. It can work though if luigi acts as a wall and lets you do stupid IC things OR if you just blizzard wall and let luigi run the stage.
That's about all I want to do right now, I can talk more about other characters too, but these are the most viable characters that you see most often in tournaments.