Why 'they seem fine together' is the dangerous sentence
Cats and dogs are predators and small pets are prey, and that wiring does not retire because an individual animal seems gentle. A dog with no history of aggression can switch into a chase-and-grab sequence in a second when a rabbit bolts, and a cat's casual paw through cage bars can hook a guinea pig or leave a scratch that turns septic — cat saliva carries bacteria that are especially dangerous to small animals and birds. Prey animals also read the situation even when nothing happens: a hamster whose cage is patrolled by a cat all day lives under constant stress, which suppresses immunity and appetite. Safety here is designed separation, not supervised friendship.
Rooms, doors, and cages as the real system
House the small pet in a room the cat or dog does not enter, with a door that latches properly — and a note on it if visitors are prone to wandering. Within that room, the cage itself is the second wall: choose sturdy construction a dog cannot knock over, latches that need thumbs rather than a nudge, and bar spacing narrow enough that a cat's paw cannot reach through to the animal resting inside. Site the cage off the floor on a stable stand where possible, away from surfaces a cat can launch from. For rabbits and guinea pigs with exercise pens, the pen goes inside the closed room, never as a barrier between prey and a loose dog in the same space.
The rules for handling, children, and visitors
Most accidents happen during handling, not housing. Small pets are held and carried only inside their own room with the door shut, never transported through areas where the dog is loose, and never placed on the floor 'for a moment' outside the safe room. Children need the blunt version: the hamster and the dog are never introduced, not even through arms, because a squirming prey animal dropped in front of a dog is the classic fatal sequence. Visitors get the same one-line briefing when they arrive, and cage rooms stay shut during parties and deliveries when doors stand open and attention wanders.
Reading stress in the prey animal you already have
If your current setup has allowed more contact than this guide advises, watch the small pet for the cost: reduced eating, hiding through active hours, freezing when the cat enters, thumping or alarm calls in rabbits and guinea pigs, and bar-chewing or frantic escape behaviour in rodents. Those signs mean the boundary needs strengthening now, not monitoring. Move the cage, close the room, and give the prey animal visual shelter — hides and a partially covered cage — so it can choose not to see the predator at all. A calm prey animal eats, explores, and sleeps in the open; use that as your measure.
Making the boundary survive everyday life
Separation systems fail through small exceptions: the door left ajar during cleaning, the dog let in 'just while I change the water'. Write the household rules down — which room, which door, who may handle, no introductions ever — and treat near-misses as the prompt to add a self-closing hinge or a second latch rather than a telling-off. Behind that latched safe-room door, KinsLe Family Sharing gives relatives you select access to its profile.