Thanks to taillow’s flocking instincts and the fact that they were already integrated with one another previously, you will likely not need to worry about these two fighting. You may need to aware of possible tension later on in their respective evolutions because of swellow’s protectiveness compared to talonflame’s preference for solitary hunting, but by then, they should have spent enough time with one another that resolving this issue shouldn’t be too difficult. Tamed pokémon generally do remarkably well together if they’re taught from early ages to see one another as teammates, rather than members of opposing species, and this is something every good trainer has some concept of already.

The only thing you’ll really need to worry about is space. Both of these pokémon will need large cages by default, but as they evolve, they’ll need more and more area to fly about in. Talonflame especially need vast swaths of territory, especially given the fact that it will very likely want to exercise its signature fire abilities. If you’re a traveling trainer, you may have all the space you’ll need for your talonflame to feel comfortable, but domestic situations may be a bit more challenging. It’s best to give your fletchling or fletchinder an everstone to avoid the problem altogether, but if your fletchinder insists on evolving, move your talonflame’s roost outdoors.

Swellow, who are more used to nesting, may be fine indoors as well, but bird pokémon generally thrive better in fresh air. Yours may be just fine sharing a space with your talonflame, however—again because its flocking instincts will drive it to share space with other birds. The only thing you would need to add to this outdoor home would be a nesting box for your swellow, especially if the swellow is the female while the talonflame is the male. (Be aware that the two may be tempted to mate. If you allow this, then the box will be necessary. If your swellow is the male in the pairing, your talonflame can build a nest anywhere. Simply provide her with materials and be aware that “anywhere” may mean your roof. If they’re the same gender, then this point is moot, although your swellow will find comfort in the box anyway.)

As for general care, mirrors are certainly a good idea to keep them occupied, as are jingling toys, ropes, and different levels of perches. Always invest in heat-resistant toys for rather obvious reasons. You will also need a sand bath for your fletchling if it evolves. If it doesn’t, it will do just fine in a water bath alongside your taillow and/or swellow. Both need sources of drinking water either way. Additionally, both have similar diets and can eat seeds as well as dried insects if they don’t eat kibble. (However, be aware that your fletchling may also prefer worms—available at pokémon bait shops—as well as larger chunks of meat in its talonflame stage.) As for lining, you can line their cages with shredded paper and untreated newspaper. This is true even as fletchling evolves, as a fletchinder’s and talonflame’s droppings do not, well, come out on fire (unlike a number of other fire-types’ excrement).

Best of luck, anonymous!