Landorus and Gyarados can actually work really well together. Because they don't share any common weaknesses and have a common STAB, you can pair them up as either a defensive or offensive core or one of both. Lando will almost always run Sheer Force with Hurricane, so you can pair it up with Intimidate Defensive Gdos, or an offensive Gdos, Therian can run both physically offensive or defensive which means it can be paired up with a specially defensive Gdos as well. The one area where they begin to differ is that Landorus-T has access to both hazards as well as Roost, and a sub-breaker in bonemerang, whereas Gdos gets Waterfall instead and a typing that isn't 4x weak to priority, resists most priority outside of Extremespeed. It depends on your playstyle.
Also I don't see why they'd be competing with Swellow as a flying type. Swellow's frailer than both of them and Ursaring, its flying movepool is more or less the same, and unlike Ursaring it doesn't get a fighting move (but could probably work around that with Nature Power). That being said, I don't think it's going to KO steels like Metagross even with guts because of its base 85 attack, and it will die to any priority anyway.
Now, Yanmega and Thundurus, I agree. I alternate between him and thundurus, since even with a better typing thundurus dies to ice attacks anyway. Yanmega, I've only used him once, and the few times I faced him I managed to spore him, but he's dangerous when he sets up and isn't outsped. Definitely a threat, but needs massive support that Lando probably doesn't need.