Possible but Not Guaranteed
A ferret and a Cavalier King Charles Spaniel may get along, but it depends on their personalities and how they are introduced.
Potential Issues
Predatory instincts, size differences, and personality clashes can cause problems.
Increasing Harmony
Use gradual introductions, positive reinforcement, separate spaces, supervised playtime, and obedience training. Never leave them unsupervised, and consult a veterinarian for guidance.