I saw the preview. As most of you guessed, Wallace is showcasing Milotic. Whether or not he shows his others is still in the air. He is only seen using Milotic.
Ash and Dawn are shown training with ALL their Pokemon. Buneary bounces on Dawn.
I don't recall a lot of the Japanese I took in school, so I couldn't tell you what was said in the preview. Dawn and Ash narrated.