The only real reason to keep the Aurochs are if you plan on re-recruiting Wakka (this can be done later in the game (SPOILER)when you ride on the Airship for the first time.) His special move gives a bonus based on the SH of all the original Aurochs (Datto, Letty, etc.) in your team. Even if you're not, there's no reason to wipe your whole team; you should replace Botta and Keepa immediately, though, unless you like losing badly. Jumal is the best goalie you can get at this stage, and Zev Ronso is versatile, with high SH, PA, and BL. Finally, get Shaami, Biggs, or Wedge so that you don't rely on Tidus to score (Datto is the worst forward in the game). If your line-up should look something like this:

Tidus
Shaami/Biggs/Wedge
Zev Ronso
Letty
Jassu
Jumal

...then you'll be fine. When you get each of these players, replace Jumal with Kyou, Letty with Linna, and Zev Ronso with Brother.