
Cavaliers are probably the best breed to have with another. I have only ever seen a couple that had been "off" with other dogs & one of them was abroad
As they roughly the same size as well there should be no problem, lots of cavalier people have other breeds like GSDs, Paps, Jap Chins, etc
If you can find a good healthy puppy go for it you will have your hands full but the older dogs will help with house training the little one