[Here is the answer for 1, 2 and 3.][1]
As for the 4, I have to say yes, from my experience: I had Haban Berry Swords Dance Garchomp, and was up against Red. I got upset when I say Red's Blastoise outspeeding and spamming my whole team with Water Spout. I saw how is it possible. It said its maximum speed can be as high as 148 at level 50, against 155 for my Jolly Garchomp. Blastoise - I deducted - was Timid or a speed-based nature with a Scarf, which makes it sped go as high as 217, and 217 is superior to 155, I presume.
The third point: Did you mean "Beat" or "Battle" them ? If you want to beat them, just a couple of OU team in a decent synergy and role distribution will be enough. Myself have been using the following:
- Garchomp
- Jolteon
- Espeon
- Arcanine
- Dragonite
- Spiritomb
and I lost 18 battles against 145 wins for 175 battles. If you did mean "Battle", then you cannot chose who will you battle.
|Post-Scriptum|: In Rental Master Tournament, never expect to use legendaries, nor your opponents. They are only available to japanese version. I saw it somewhere but can't remember where exactly. As for Mix Tournament, my advice is to use a circular typing, like Fire-Grass-Water, so whatever they pick, you can counter it. The Pokémon they give you are damn weak. Choice Band Eelektross, Sash Starmie ... These are good stuff, but they are freakin' lacking movesets. Starmie can't spin, for example.
Hope this helps.
~ Sincerely yours, Blitz of Justice.
[1]: http://bulbapedia.bulbagarden.net/wiki/Pokémon_World_Tournament
