People often ask about good companions for West Highland Terriers. Can a Westie Terrier get along with Guinea Pigs as an example.
Well, I have no idea anything about how a Westie would get along with an Iquana or a Guinea Pig, but I've got a wealth of knowledge about Westies and cats..
When Willow came to us last summer one of the things that she needed to adjust to was a household already inhabited by three tomcats ranging in age from four years to around ten. The cats definitely rule the roost here and the new little Westie just had to find her place. She's done that now. There can be occasional spats to decide who's going to be top dog/cat on any given day, but that's normal and the cats jockey back and forth amongst themselves whether the dog is there or not.
We weren't just dropping a dog into the midst of a lot of cats who had never seen a dog before. Our Norwegian Elkhound, Saladdin was a key part of our family structure for a long time and actually helped raise two of our cats from kittenhood. He's gone now and the cats haven't had to cope with a dog for quite a while, but the old habits were still there. They weren't overjoyed to acquire any kind of dog to contend with, especially an energetic pup, but it wasn't the end of the world for them either.
One thing that Marilyn and I insisted on from the beginning was that this new dog was required to submit to the cats. This is especially true when handling terrier dogs such as the West Highland Terrier. Regardless of how lovable and cute they can be, deep down within their heart and soul they are hunters. They want to get in and duke it out with the game animals, and practically anything that runs from them qualifies as game. It does not mean they aren't great dogs, but there is no way around it: the terrier breeds from Pit Bulls all the way right down to Yorkies are scrappers. So our job as owners of terriers is to define the things they can chase and fight with and what they can't.
This is why we were so strict about making Willow submit to the cats from the very beginning. It wasn't hard to accomplish. If she got too frisky we were required to lay her down until she settled and calmed, but mostly a sharp word and a tug on the leash was enough to complete the task. It certainly helped that our cats weren't particularly afraid of dogs and don't mind demonstrating the use of their claws if she gets too feisty.
Things have settled into place now and we've been able to relax a lot of the boundaries we laid down originally. We have chases in the house now, up the stairs, around and down. Its equally likely for Willow to be chased by a cat as it is for her to chase one. In truth it appears to be more of a game of tag than anything else, and nobody gets hurt. Whenever they aren't chasing one another around the house its not unusual to find Willow curled up with one of the cats, or simply hanging out on the windowseat with one. And, you can actually never tell which two animals will bond. Our oldest cat, Whiskey, was the very first to actually chase Willow (I think she pissed him off, to tell you the truth) and set the stage for the others to follow.
Your circumstances may be quite different from ours, because our cats were already used to having a dog around. If your cats haven't had anything to do with dogs, you will probably want to follow a slightly different course. In a case like that I recommend keeping the new Westie on umbilical for a couple of days while the critters get used to one another. Umbilical training, where you have your dog on a long leash and keep the leash tied around your waist, is an extremely good training method under any circumstance. It gives you control of the situation without having to hold the leash, or even pay much attention to the dog. Stick with the umbilical work for at least a week and keep the dog in its crate when it isn't on the leash. The cat will quickly settle down once it realizes that the dog is not really a threat to it.
Precisely the same things apply if you wish to introduce a cat into the house. I'd recommend a kitten if possible, mainly because they're more malleable which will make the relationship more managable. Make sure that the dog and cat get to sniff each others butts so that you have the nose engaged, then keep the dog on umbilical for some time.
Cats can definitely coexist with Westie Terriers. They will often not become buds the way our animals did, but peaceful respect for each other should be possible. Make use of the umbilical, make the dog submit at first and all should be well.