Yes, the more the merrier
No, too many legendaries
None. Game Freak can define Legendary Pokemon however they want, since it's their games, and they haven't really given a concrete definition of Legendary Pokemon. And even if they had, as it's their game, they can continue to tweak and mess with that definition as much as they see fit, as the game series ages. They're the ones that choose where to put down the boundaries, and since they aren't exactly letting us see fully where the boundaries are, it's pretty difficult and would be presumptuous of me to say that they put them in the wrong spots.
Heatran)? In GSC, which introduced genders, all Legendaries had genderless. RSE broke the patten by giving Latias/Latios genders. This was inconsistent with the previous "traits of legendaries" up to that point, which would make them just as much not Legendaries as Heatran is, under that logic. If you're actually recognizing the fact that the traits of Legendaries can be shifted over generations, as you are by calling the Lati@s Legendaries, then you must also recognize that this can continue to be done in successive generations, and due to this stuff like Phione and Heatran can be considered Legendaries just fine. Not liking particular Legendaries if fine, but just state that if that's the case, and don't try to argue certain Pokemon out of being Legendaries by appealing to certain traits of them being defined and unchangeable, while at the same time saying that others are Legendaries because... traits can change, over generations.
Heatran has a connection to Stark Mountain. That's good enough for me. It's more than what some of the original Legendary Pokemon had, gamewsie... Namely, the Bird Trio (3/5 of the original Legendaries). They were pretty much just put into the Seafoam Islands, Power Plant, and... Victory Road (don't know why they didn't just put Moltres into the Cinnabar Manison or at least something vaguely Fire-like, as they bothered to put the other two in themed locations), and were pretty much... well, just there. Their original Pokedex entries aren't really that much better than Heatran's either, beyond directly referring to them as Legendary Pokemon. Other than that, they just said "yeah, this Pokemon's a Legend, and here's a random tidbit about it, just like most other Pokedex entries" instead of really focusing on that Legendary aspect. Thankfully, we ended up getting the second movie to help elaborate on the birds (although, their appearances within the anime itself since then have been mixed--sometimes they come off as actually being Legendary, like the Moltres in the Indigo League did, and other times they just seem more "rare and powerful Pokemon", like Noland's Articuno)... But within the games themselves, they pretty much just seem to be in the same boat as Heatran to me, which really makes it interesting to me that people seem to complain about Heatran the most out of the more recent Legendaries, when it seems to me to be most like the original Legendaries.
The more the merrier. :P
What's with all the Heatran hate?
Heatran, it has both genders, but that's also the only reason to remove it's status as a legendary.
Mewtwo. How can a Pokeon created by Humans only recently be considered Legendary?
How long something have been a legend is irrelevant for the fact that it is a legend or not, you are apparently "legend making" when you conquer Kanto in HGSS (don't know about GSC), thus you are consider a legend in Pokemon World.
I was saying Legendary as in Mythical which is what all the other 'Legendary' Pokemon are.(Except Deoxys, maybe)