In order for me to answer your question I need a little information.
What breeds and sexes are the other two dogs?
Hope to hear from you soon,
Are your other dogs Rotts also or another breed?
I do not know how much you know about Rotts. They can be a fantastic dog with the right owner. I would suggest that before deciding on one, to go to a dog show and meet a breeder and really get to know the breed. You can find upcoming dog shows by going to the AKC website and finding the show calendar: http://www.akc.org/ Make sure you really look at the temperament of the mother of the puppies. You might not be able to meet the father. Hip Dysplasia is a real concern, so your pup should come from OFA certified parents. A good breeder will be proud of their dogs and eager to show their health certifications as well as show awards. I think it is a great idea if you get the female spayed and your Rott and Rat Terrier neutered. Rotts often do not get along well with others of the same sex, so if you have a female outside with him, that will work. I think a Golden with their more relaxed temperament will be a good match for your Rott. More then likely you will be getting a puppy so he will grow up with your other dogs and cat. Most dogs do well with cats if they are raised with them. I am a bit concerned about the eventual size difference in the two males. Rat Terriers have not figured out how small they are (which is part of their charm, I know, I have a mix). But if you get your Rott neutered young, at 6 months and get the rat terrier neutered it will be less of a problem as will the Rott being a puppy with the older dogs and growing up with them. I would say that an obedience class should be on your to do list. Rotts even neutered ones are going to test you and want to be alpha, the pack leader. When your Rott becomes and adolescent it will be even more important for you to keep watch for any aggressiveness with your Terrier. Be prepared to correct him and let him know that fighting is not allowed, ever.
I hope this has answered your question. If it has not or you need more information, please write me back and I will gladly respond. Feedback is also very important to me, I want to know what you think. If my answer has answered your question please hit the Accept button. Bonuses are always a wonderful welcome.