Two shocks, handled one at a time
A small shop fish arrives stressed twice over. The first shock is chemical: shop water differs from yours in temperature and makeup, and young fish have less reserve to absorb a sudden change. The second shock is social and physical — your adults are bigger, established, and in some cases quite capable of eating a newcomer that would fit in a mouth. Acclimation done well separates the two problems: match the water slowly first, then manage the introduction as a size-and-shelter problem, not as a single hopeful tip from bag to tank.
Float first, then drip
Float the sealed bag in the tank for fifteen to twenty minutes so temperatures equalise gently. Then open the bag, roll its top down so it floats, and begin adding your tank water a little at a time — a slow drip through airline tubing is ideal, or small cupfuls every few minutes — until the bag holds at least twice its original volume. This staged dilution is kinder to youngsters than the old float-and-tip, because organs stressed by a long bag journey handle a gradual change in water chemistry far better. Keep the tank lights off throughout: darkness calms the residents, calms the newcomers, and stops the adults treating the bag as a feeding event. Net the youngsters out into the tank rather than pouring bag water in with them.
Release like a small fish
Where you let go matters. Release the youngsters low, beside dense planting, rockwork or a corner thicket — never in the open middle of the tank where every adult can inspect them at once. Feeding the adults at the far end just before release buys the newcomers a distracted first minute. Then let them be: a young fish that dives for cover and stays there for the first hours is behaving sensibly, not failing. What you want by the next day is exploring in short trips from shelter, returning when chased, with no adult singling one out for sustained pursuit. Feed at the youngsters' end of the tank for the first days, so they eat without crossing open water, and if one individual is singled out again and again while its siblings settle, take that as information: that fish goes back to the nursery to grow, however well the others are doing.
The size question decides more than the method
Be blunt with yourself about the mouth rule: if a youngster can fit in an adult's mouth, someday it may. Community adults that ignore tiny tankmates for months have eaten them during a hungry night. For genuinely small juveniles, the safe route is a mesh breeder box hung inside the main tank or, better, a few weeks in a small nursery tank on the same water until the youngster outgrows the danger size — acclimation then becomes trivial, because the fish is already living in your water and only the address changes. Whichever route you take, the settling-in record starts now: shop, date, size and first-days behaviour noted in each youngster's KinsLe Pet Passport means that weeks later you can judge its growth and health against a written start, not a vague memory.