Prey-sized fish in a house with hunters
Fork-tail rainbowfish are good with other pets at home only in the sense that a well-covered aquarium keeps them apart. A fork-tail is about two inches of slow, glittering movement, which is precisely what a cat is built to notice. The fish itself is peaceful to the point of vulnerability and will never trouble another animal. Every part of this question therefore turns on barriers and tankmates: a lid a cat cannot shift, a stand a dog cannot rock, and companion fish small and calm enough that the fork-tails are not bullied, outcompeted, or eaten.
Cats and dogs around a fork-tail tank
A cat will watch a fork-tail school for hours, and that watching is harmless until paws meet water. Fit a close, sturdy lid with no open feeding hatch left ajar, because a cat that dips a paw once will try nightly, stressing the school and risking contamination. Position the tank so the cat cannot sit above it and dangle a paw through gaps around cables. Dogs are a simpler matter: most lose interest quickly, but a dog that jumps at the glass can terrify a shy school and, in the worst case, crack a small tank, so train a settle spot away from the aquarium and keep the stand solid. Caged small pets and birds in the same room pose the fork-tails no problem at all.
Tankmates that are safe for fork-tails
Inside the water, size is the whole rule for fork-tail rainbowfish. Safe companions are other small, peaceful fish that share soft, warm water: small rasboras, ember-sized tetras, pygmy corydoras, otocinclus, and peaceful shrimp are typical good company. Anything large enough to fit a two-inch fish in its mouth is a predator here, however peaceful its reputation, and that includes adult angelfish and larger rainbowfish. Fin-nippers and fast, greedy feeders are the second problem, because fork-tails feed delicately at the surface and a school mobbed by bigger appetites simply gets thinner while the tank looks well fed.
Adding new animals without unsettling the school
Introduce change around fork-tail rainbowfish gradually. A new cat or kitten in the house will make the school shy for days; keep feeding times and lighting exactly as usual and let the fish learn the new shape outside the glass is not a threat. New tankmates should be quarantined where possible, added in the evening under low light, and watched at the next several feeds to confirm the fork-tails still reach the surface food. Adding extra fine-leaved planting at the same time gives the school refuges while the new social order settles, which prevents the chronic hiding that slowly starves a nervous fish.
Signs the current mix is harming fork-tails
A fork-tail school that no longer leaves the plants, shows nipped tail edges, or grows thin while tankmates thrive is telling you the mix is wrong, and the fix is to move a species, not to feed more. A cat found repeatedly on the lid or with a wet paw needs a latched lid or a closed room door, because one successful grab ends the question permanently. If several fork-tails die shortly after a new tankmate arrives, test the water and consider disease carried in with the newcomer, and get advice from an experienced aquatic shop promptly while the rest of the school still feeds.
Safe when the barriers do their job
Fork-tail rainbowfish can live happily in a multi-pet home, but they are the animal most dependent on the keeper getting the barriers right: lidded tank, steady stand, and companions their own size. Do that and cats become an audience rather than a threat. If the school is ever passed to a new keeper, KinsLe Transferrable Profiles records which tankmates the fork-tails lived with safely and what tiny foods they were fed, so a delicate balance is not undone by a new owner's guesswork.