Dogs are like people; each has his own distinct personality. That is why you may get a dog slightly different than the breed standard with regards to temperament, though the breed standard is a very good indication of the AVERAGE characteristics of a dog of that breed. Breeding and training also play a very important role in the dog's behaviors; dogs from reputable breeders will resemble the breed standard much more than those you find at pet shops or from "backyard breeders." If a dog isn't trained, his behavior may diverge completely from the breed standard and all the reasons why you chose a particular breed will get thrown out the window.
In general the Welsh Terrier is moderately territorial, not overly aggressive, but hard-wired to hunt and kill vermin; furthermore, he was developed on farms, around chickens, horses and other animals. This translates into a dog that is potentially tolerant of other animals, but has an instinctive drive to chase small, furry, fleeing creatures. Actual Welsh Terriers will deviate from this average, in either direction. There are some Welshies that have no problem with any other animal they encounter and actually LOVE to make friends, while others can be very intolerant of other animals, trying to start fights; still others will not start fights but will be very easily provoked by other animals. Once a Welshie feels threatened, he will not back down. There are certain Welsh Terriers that engage in frequent dominance displays, especially towards other dogs.
There are Welshies that are EXTREMELY territorial and do not tolerate the presence of any other animal visiting, let alone living in their house; some consider their owners as fundamental parts of their territory and become aggressive towards other dogs that get close to their owners while on walks. Most will attempt to chase squirrels and rabbits, while some will bolt off after anything that moves (including leaves); some will also try to chase after your or your neighbor's cats. Pets like hamsters and guinea pigs may not be safe around certain Welshies, while others may completely ignore them; the prey instinct is not equally strong in all dogs. Indeed, there are quite a few Welshies that are disqualified from Earthdog trials because of their total lack of interest in chasing rats down a hole.
Whatever the personality of your Welshie, socialization is the key to moderating any potential intolerance of other animals and/or pets. If you'd like to have more than one dog in the household and if you'd like to eventually add other pets, you should get a Welsh Terrier puppy and begin exposing him to other animals as soon as you bring him home. Let him play around any small furry creatures you have, like hamsters or ferrets, to get him thinking that these little animals are part of his family and should not be chased. Even with a properly socialized dog, though, you should never leave him unsupervised with your smaller pets, or accidents may happen.