Considering that the game was being developed on the N64, and he worked as a second party developer for Nintendo, Sakurai more than likely had access to such information. Anyway, popularity isn't the only factor as I've said before.
Maybe not, but in the case of Lucas back then, it would have only been for marketing reasons. If a character is unknown, doesn't have any established history, and much of their moves would have to be borrowed from another character, then its pretty safe to assume that the only reason they would be included is for the sake of being a advertisement. Since Earthbound 64 didn't come out in time/at all really, Lucas didn't make the cut for Melee, even though he was intendded to.
If Link and Zelda were strictly their TP incarnations, we would have had very different movesets. Heck, we wouldn't have Sheik in Brawl either. While the Adult Link is used only in OoT and TP, I think he's considered the standard Link. We didn't see Young Link in SSB after all.
Why's that? TP Link technically shared more moves with Brawl's version of Link than OoT Link did with Melee's Link. Why would he have a very different moveset? And while I would say that Zelda's moveset doesn't really reflect her TP incarnation, in reality it doesn't reflect any of her incarnations, since she has never used any of those abilities. (Technically Sheik's the same there too) And Adult Link is only the most popular, and likely iconic Link, but he isn't the standard Link, since that's not how Link is used the majority of the time.
Technically, Roy doesn't. If FE was subject to "topical" (I think this is a poor word for how recent something is.), then Roy wouldn't have been the second closest character to making it in Brawl. Heck, he wouldn't have been one of the Seven Deadly Fighters in the first place.
That doesn't make sense though. While its debatable that his popularity almost got him into Brawl, that still doesn't change the fact that his initial inclusion was simply a advertisement. (Nor does it change the fact that Roy didn't get into Brawl) While topicality isnt something that has to be completely followed (for instance Jiggs isn't topcal yet she gets in) it does seem like a major factor for incoming new characters, and it does seem to effect pre-existing characters to a degree, such as the LoZ characters who change from game to game, yet are pretty much the same.
So what if the first two gens of Pokemon were taken from the last smash games? That still doesn't explain third gen Pokemon like Gardevoir.
Why would that need to explain Gen 3 Pokes? Melee never represented them, and they were still somewhat topical. For mostly all of the preestablished series, the content was taken from either the very begining of their series, or from the games that came out between Melee and Brawl. R/S/E came out between the two games, and thus got represented in Brawl, even if they weren't the newest thing, but Gen 1 and Gen 2 were covered before and thus got punked a bit.
Pluuus, I think its interesting to note that mostly all of the Gen 4 Pokemon were ones that were advertised before the games actually came out, indicating that Sakurai's team probably didn't have much access to the plans for Gen 4. The only Pokemon who I think really breaks this rule (aka wasn't just a trophy that could have been put in last minnute, or some hovering thing in the background of Pokemon Stadium 2) was Cresselia, who doesn't really make sense.
