At what percent can I land a perfect combo with Captain Falcon

No, you cannot trade trophies. There's no such feature anywhere in the menus, this capability was not mentioned in any pre-release Miiverse posts or Nintendo Directs, and the previous game by the same team, Kid Icarus: Uprising, had similar collectibles (Idols) that also could not be traded between copies of the same game, despite there being other collectibles in that game (Weapon Gems) that were able to be shared via StreetPass.

You will eventually be able to transfer customization aspects (or at least some of them) with the upcoming Wii U version, but because many of the trophies in each version are unique to that platform, trophies cannot be transferred between versions, either.

Going by the example set by previous games, training mode defines a "combo" as "series of attacks that keep the opponent in hitstun", possibly with one frame of escape window (not sure as of this time). So, you need to get in 100 hits without allowing the opponent a chance to act.

Being that it's hard enough to get a legitimate combo of 20, you're going to have to cheat a little bit. Spawn a bunch of Smart Bombs and explode 'em. Depending on a few migitating factors, the resulting combo will be somewhere between 50 and 100. Give it a few tries and you'll get it eventually.

As for the "400 across everyone" challenge, that's an average of about 8 per character. The chunk of 100 will drive this down to about 6 per character, which is easy enough with a single Smart Bomb or even characters' infinite jabs.