We're taking in an older dog on Friday, totally unplanned. We had originally wanted to wait until we no longer had our current pets to get a new animal so we wouldn't stress them out but it's an unexpected thing (I don't want to go into all the specifics). We currently have a 10 year old dog and a 9 year old cat. I have absolutely no idea how they're going to react to a new animal in the house (the new dog is almost 8 years old). Any advice on making sure this goes as smoothly as possible?

Also, any advice on housetraining an older dog? We have an appointment with the vet on Friday afternoon and they said they would give us some advice on that, too, but if anyone has any advice from experience, it's definitely appreciated!