People who care about characters in a Super Smash Bros. game are generally people who follow this game. I can tell you right now that most people did not expect a 6th gen Pokémon, let alone expected it over Mewtwo/Lucario. By 2013, most people were expecting Brawl + Mewtwo. When Smash Direct revealed that Gekkouga would be in (by that time, Lucario had been confirmed, so people had even higher expectations for a Brawl + Mewtwo scenario), most people were shocked at the fact that a Pokémon X & Y character actually made the cut, let alone Gekkouga.
Brawl + Mewtwo had definitely more to it than just personal wants backing it up, there were very good reasons to expect it. Everyone in this hypothetical scenario had a very good reason to return. Pikachu was the mascot of course, but the others made sense from being relevant.
Jigglypuff: Obtained a new type.
Pokémon Trainer: Starred in the Pokémon Origin special.
Lucario: Remained strongly relevant even after no longer being the mascot of the current gen; helped that he got Mega Evolution.
Mewtwo: Most prominent it has been ever since the 1st gen (obtained a new form, played a major role in Genesect's movie, etc).
Combined with Mewtwo being the most wanted character for this game (even above that of Mega Man, who got in because of popularity) and it made sense to not expect a 6th gen Pokémon from a logical viewpoint. We didn't need a 6th gen Pokémon to fulfill any quota, Brawl + Mewtwo could do it from a popularity and relevance viewpoint.
Very specific and 99.99% impossible to predict scenarios being correct have pointed towards the leak being correct before.
Take
ChaosZero's leak pre-Brawl. He leaked various informations about characters; such as Lucario getting in over Mewtwo (not necessarily as a replacement, just higher priority), Wolf being in, etc. He also leaked a very specific piece of information; the Dragoon item where if you collect three parts, you would use them as a devastating weapon. This was something that no one had predicted before. Once the Dragoon showed up on the Smash Bros. Dojo website for Brawl and as exactly as ChaosZero described, it was obvious the leak was legitimate. People denied it furiously even after this piece of information was revealed true, but everything ChaosZero said was right. Now we see people doing similar things with Wii Fit Trainer, which was equally as difficult to predict (aka no one).
If by "fan expectations" you mean desire, then that is incorrect. Of course Sakurai cares about fan desire, he's not just going to ignore them entirely. This is the same man that added Mega Man, the second most wanted character for this game, because of fan demand. He did the same with Sonic during pre-Brawl as well. Sakurai has also
made it clear that while not everyone from the previous games (including Melee and Brawl) will be returning, he does understand that fans do care about the characters that are in the game and will do his best to keep them in. While Sakurai is not going to grant every popular fan request under the sun, it doesn't mean that they have no bearings on the roster selection.
Salromano's tipper likely obtained the information in the design document while the game was very early in development. It explains why Villager was "Animal Crossing Guy" instead of a Villager and why we have "Pokémon from X & Y" instead of Gekkouga. With "Pokémon X & Y", it may be that the Pokémon Company and Sakurai had added a spot for a "Pokémon X & Y" at the start yet had not decided on a Pokémon newcomer. Once X & Y came out, marketing research was quickly done to see who was the most popular Pokémon and Gekkouga turned out to be the best choice. Gekkouga was then added late in development, but because a spot for Pokémon X & Y was planned, this was made possible. Dinoman pointed out that a similar thing happened with Ike in that he was not planned from the start as a space for Fire Emblem had added from the start. However, Ike was eventually deemed the best choice and was added.
As for Rosalina, Salromano's tipper may not have been able to obtain all the information for planned characters in the design document. Not once was it claimed that these were the entire planned additions for this game, just that these characters were in. It could be that there are other characters that Salromano's tipper may not have been able to find out about; one of those being Rosalina.
Also, I never said that Pokémon X & Y made the leak more credible; my entire argument has been that it does not discredit the leak, which is not the same thing. I also fail to see how Salromano's tipper not conforming to fan speculations for leaks and seeing what he saw is a point against the leak.