Read the older dogs honestly
An older dog who moves away, turns her head, tucks herself behind furniture or gives a hard stare is not being grumpy — she is asking for space in the only language she has. A young dog, especially in the bouncy adolescent months, often misses or ploughs through those signals. If the older dog's polite requests are ignored long enough, corrections escalate from a growl to a snap, and the youngster learns that other dogs are unpredictable rather than learning to read them. Your job is to intervene long before the older dog has to shout.
Separate by default, together by invitation
Use baby gates, closed doors and separate resting areas so the household default is apart, not a constant free-for-all the older dogs cannot escape. The young dog spends much of her indoor time in her own area with chews and toys, while the older dogs keep the sofas and quiet rooms they have always enjoyed. Time together then becomes a deliberate, supervised event — a shared walk, a calm scatter-feed in the garden, training turns side by side — that ends before anyone is frazzled, rather than an endurance test.
Spend the youngster's energy away from her elders
She still needs to play hard, wrestle and be silly; she just needs partners who volunteered. Take her out separately for training, play with you, and meetings with a suitable young playmate. Tire her brain with searching games and short training sessions before any together-time, because a young dog who has already worked and played arrives far more capable of settling. Expecting an older dog with sore joints to be the puppy's entertainment is unfair to both: one is over-aroused, the other is quietly miserable.
Guard the flashpoints: food, chews, beds and doorways
Most multi-dog friction ignites around concentrated resources and tight spaces. Feed everyone separately with a barrier between, lift uneaten food, and give long-lasting chews only in separate areas. Give the young dog her own bed in her own zone rather than letting her flop onto the older dog's bed for a reaction, and manage narrow doorways by letting the older dogs through first without a youngster barging past. If an older dog ever guards seriously, or a scuffle draws blood, separate fully and ask a vet or qualified behaviour professional to assess the household in person.
One plan that every person in the house follows
The arrangement collapses if one person enforces gates and rest while another lets the youngster charge in because it looks cute. Agree who exercises her and when, which rooms are older-dog-only, and how together-time is started and ended, and stick to it on tired evenings too. As she matures and genuinely learns to read a turned head and walk away, you can relax the separation in stages, rewarding every calm choice she makes near her elders. For this youngster, Family Sharing in KinsLe opens her profile to the relatives you select.