What breed of dog is compatible to fox terrier?

A fox terrier is classified as a terrier so is relatively small in size (approx. 40cm) so I would recommend a dog of a similar size and temperament. Here are some breeds:

Norfolk Terrier: This dog breed belongs to the terrier group and is approx 25cm tall. They are intelligent and playful dogs that get on relatively well with children and animals especially if introduced as a pup. They are lively and enjoy exercise but have a tendency to bark and dig.

Dachshund: This dog breed comes in 2 sizes (Miniature; 16cm Standard; 23cm), and has 3 different fur coats (wire haired, smooth haired and long haired). This breed belongs to the hound group but has the temperament similar to a terrier. They are active, playful and highly intelligent but do not need much exercise. They are easy to transport and affectionate with their family but have a tendency to be more wary of strangers.

Poodle: This dog breed is in the category Toy dogs which means they are bred for companionship. They come in a variety of sizes and I would recomend miniature (approx. 40cm). Poodles are easy to train and get on very well with other animals and children.

Other breeds you may wish to consider are: Border Terrier, Cocker spaniel, Schnauzer, Jack Russel terrier, Pug, Yorkshire terrier or maybe another fox terrier.

Hope this helps :)