Sootopolis, and Kyogre is located there too :-).
In the elite 4 hes the champion
I don't know!Wallace appears in Ruby Sapphire and Emerald.Most likely from Emerald but there may be some elements from Ruby and Sapphire.
Wallace he was the former 8th gym leader in ruby and sapphire
I'm sorry to say it, but "that guy" is not a Pokemon. He is, however, the leader of the Sootopolis Gym. In Ruby and Sapphire, he was Wallace. But in Pokemon Emerald, he became Juan. Wallace then became the Elite Four Champion.
If you mean "Is Juan the leader in Pokemon ruby?", then no. It's Wallace who's the leader. Juan is leader only in Pokemon emerald. The champion is Wallace in Emerald, but Steven is the champion is Ruby and Sapphire.
the same Pokemon as sapphire sealeo luvdisc whiscash seaking milotic
When you win Elite 4 and the Champion Wallace/Steven.
Defeat Wallace in sootopolis city, you can only come there by using dive.
you have to go to sootipoolis city which is next to mossdeep city
It's because on Pokemon Ruby and Sapphire, Steven is the league champion and on Pokemon Emerald Wallace is the champion instead of the final Gym Leader. Juan doesn't exist on Ruby or Sapphire.
No, you only battle Steven because in Pokemon emerald Wallace is the champion.