It's pretty obvious why Zangoose and Seviper hate each other. It's because Zangoose is a mongoose like pokemon and Seviper is a snake like pokemon. Mongooses and snakes (well, corbras at least) hate each other.
I think this is a good episode. I laughed at some parts (Meowth replacing Jessie, everyone chasing after Woobufett, Jame's comment about Nicolai). But I didn't like the trainer's choice or Zangoose's voice.
I'd give this episode an 8.5/10