On the subject of philia, the brotherly love.
It is beyond doubt that, as many have professed above, Marth and Captain Falcon would be fast friends. Yet, such would seem the case of nearly any hero pair. Lucario and Meta Knight, Fox and Mario, King Dedede and ROB, no doubt, and all the in-betweens, such as Mario and Falcon, or Lucario and King Dedede.
The only questions arise when there are 'interesting' characters introduced, that is, characters who don't have exclusively heroic traits, such as with Falco, or Luigi, notably the villains such as Bowser and Wario, and even the less humanoid peoples whose intellectual, emotional, or behavioral capacity is directed other ways such as Kirby or any of the Pokémon.
There is something to be said for the secondary traits of the heroes, that is, what makes them more suitable to others. For example, Captain Falcon is a mercenary, even though his primary motive in the F-Zero cups is to halt Black Shadow and the myriad of other criminals, this would lend him well to sympathize, I feel, with Fox, whose situation is not unlike his own. The difference lies in that Fox is contracted to destroy his enemies, whereas Falcon is competing to destroy his enemies. Their differences would fuel their dialogue, and they would inevitably find a friendly acceptance. Samus, likewise, would sympathize with Fox in the same way. Certainly their small talk would be filled with techno-babble as well, an easy conversation topic. These sorts of things are what forms the majority of what people view as possible pairings.
But this is the real antecedent of speculation, just as it is the antecedent to romantic speculation: the secondary traits of a character are myriad. To speculate on who they would most likely associate with is not beyond us, but we must pick and choose what we would hold influential to who would be a bro to whom.