It may be a bit difficult because sneasel are more accustomed to life on the mountain (specifically in the caves therein), as opposed to the sunny, tropical climate of the lower altitudes of Alola, but with the proper preparations, it may be possible. But first, as for whether or not you should adopt the newborn, it would be worth it if and only if you’re absolutely certain it was abandoned. Sometimes, a mother pokémon will run from its own offspring in order to draw a predator’s attention away from it, and dark-types may hide their kits and spend much of the day away from them to hunt without having to worry about their nests being raided. If you found this newborn out in the open and if the sneasel didn’t bother attacking you to draw your attention to her instead of the baby, then that’s a good indicator that the child is indeed abandoned. Likewise, if the mother hasn’t come back after a few hours, this, too, tends to be proof.

If you’re certain that the mother isn’t coming back, then yes, for the kit’s survival, it’s okay to adopt it. Once you make that decision, though, you’ll need to ensure that your home is ready. My advice would be to leave the kit at a pokémon center (which would be a good idea to stop at anyway so that a professional nurse may give it a once over to check its condition) and go home to prepare a room for it.

Remember that sneasel are nocturnal pokémon used to colder climates. Thus, you’ll need a dark room (one with curtains—the blackout sort isn’t necessary but is highly recommended—or a room without windows at all), preferably one with air conditioning or one of the cooler rooms in your home. As a note, it’s possible to invert a sneasel’s sleep cycle and train it to be diurnal, but frankly, for a growing kit, it’s best to allow it to follow its natural sleep schedule.

On that note, if you’re not nocturnal yourself, given your other pokémon, it would be a good idea to introduce the kit to your team early, especially mimikyu. As naturally nocturnal pokémon themselves, they may be able to provide valuable help for caring for sneasel even while you sleep. In particular, I would recommend teaching your mimikyu basic care skills and putting it in charge of the baby. By giving it in particular the rule of caregiver, you may be able to avoid making mimikyu jealous of the attention you give your sneasel; instead, it will feel like it’s an integral part of sneasel’s growth—perhaps even an adoptive parent. (Yes, it’s an incredibly good idea to word it as such to your mimikyu.)

Also, be sure you have a soft bed to emulate a sneasel nest, a litter box (for waste), soft toys, and plenty of formula. Eventually, your sneasel will need kibble (or meat and nuts, if you can provide them) as well as its own water dish, but as a newborn, it’s likely it will need milk instead unless Nurse Joy recommends otherwise. The soft toys are enrichment; eventually, you’ll want to move on to a scratching post and harder toys to destroy, especially as the sneasel’s claws grow in. Place these items in the room you designate as your sneasel’s, and you’ll be ready to bring it home.

Once home, the first few months will be exactly like caring for any other newly hatched pokémon. Be sure to feed it every three hours, allow it to sleep, and keep a close eye on it at first, as you would with any other baby pokémon. Follow the same schedule for weaning it onto solid foods as recommended by your Nurse Joy, and if your local Nurse Joy gives you advice on how to care for your particular sneasel, follow it.

Other than that, acclimation will of course be important. As it grows older, you may wish to expose it little by little to the world outside its room. Allow it first to adjust to temperatures by letting it explore the rest of your home, then the area around your home. If it looks uncomfortable, guide it back to its room and allow it to cool off. Make absolutely sure it gets plenty of fluids, and keep a close watch for any signs of heat exhaustion (or worse, heat stroke). If your sneasel seems disoriented or overly lethargic, or if it gets sick or loses consciousness, take it to the pokémon center. Most importantly, remember that heat is the harder of the two conditions for an ice-type to get acclimated to, so be patient and do it gradually and according to your sneasel’s comfort level. Then repeat the process for sunlight (i.e., gradually increase exposure until your sneasel is perfectly happy in sunlight).

As for alternative companions, your team may be sufficient enough, but if you wish and if you can support them, it may be helpful to obtain a member of the poliwag family from the nearby Malie Garden. These pokémon not only learn water moves naturally, but they can also be taught ice-type moves, either of which may help in keeping your sneasel cool. Additionally, meowth from the same location may be helpful in raising your sneasel, as that pokémon family shares many similarities to sneasel’s. (They’re mammals, they’re dark-types, they share similar diets, and they both rely heavily on claw-based attacks.) Be careful when obtaining partners to assist you in raising sneasel, however, as you do have your mimikyu to consider. Always be clear about your intentions, and perhaps consider working together with your mimikyu to select and obtain a suitable companion.

Best of luck, and potential congratulations to the new addition to your family!