It needs time. If the new cat is doing that, it is stressed. I would suggest at least several weeks of them being apart. Once the new cat has settled in the separate room, you could try putting it in a crate and bringing it to the room with the resident cat. It may get upset and charge again. But it will be in the crate. This is okay. Cats take time. I just went through an incident with a cat I've had for about six months. One of my older cats (14 years old) decided she didn't like her anymore, even though they had been sleeping together on the couch just a week before. In turn, this put the younger cat on her guard, and she started charging and hissing at my other cats (I have five total). It's taken about six weeks, but it is starting to get worked out. I would separate them when they were fighting (put one in a higher spot, and move one to another room). Cats take time. They do have an order. You need to take the time to let that develop as well.