It has been a while since I watched it but to answer you questions:
1. I don't think there is any Devilman animals, atleast in the various adaptation and the original manga I read/watched. I believe these people are demons that have the forms of animals (keep in mind I haven't watched the anime since last year so I could be misremembering what demons you are talking about).
2. The Babo betrayed them. He saw that Miko had the glasses of the rapper who confessed to Miko right before she and him went to the Sabbath. This lead him to assume she is a demon and that she killed his friend and ate him. The anime doesn't outright confirm that she killed the rapper but that's what the traitor assumed.
Edit: looked up the Devilman animals and remembered the cat. That's the only instance that we saw an animal Devilman as far I remember and it is only in this adaptation.
Yeah, that's what happened. It's good background storytelling. Think that's also why she helps out the track star who's a Devilman cause he did the same thing to his boyfriend.
I was just wondering about the animals, because when Satan first falls down to Earth the devils are depicted with animal characteristics in their fight for dominance. There was also Tako our one and only Devilcat, it had me curious.
Hie (the short one) is the one who saw Miko with Kukun's (Mayuta) glasses. Babo is the tall one with the panama hat. His eyes as he stabs Gabi look like he was shot up with drugs. Not too important to know in the grand scheme of things, just some of the few things I'm unclear about.