For your final pokemon I would choose between Metagross, Zangoose, and Manetric, as poison isn't a very good type(Swalot), and I've never really liked Exploud.
Metagross has very good stats and can learn a lot of good moves such as meteor mash and earthquake. However, you can't get it until very late in the game, and when you do, you have to level it up a ton until it actually becomes useful. If you're planning to take on the Frontier with this team, I would definitely go for it, as I used it on my Battle Frontier team which was able to win me all seven silver symbols. However if you just want to play through the regular game, then I would go with one of the other options.
Zangoose isn't obtainable without trading from Ruby in Emerald, so if you can't do that, he isn't an option. However, if you can get him, I would make him your first choice, since with one or two swords dances under his belt and a STAB slash and quick attack, he can sweep most boss trainers in the game. He also is very cool looking.
Manetric IS obtainable very early on, and has decent stats. His electric typing will also be useful to you in fighting the Champion, the eight gym, and is just a generally good type all around.
As for the Waillord/Walrein dilemma, I would go with Walrein, since having a good ice beam on the team somewhere helps a lot, and you don't have anyone else who can use it on your team. (except maybe Claydol or Altaria, I'm not sure. Either way, Walrein gets STAB).