Seeking perspective: integrating new 9-month-old Cocker Spaniel with 4-year-old Pit Mix

Hello! I’m looking for some advice or perspective on how to keep progressing with my two dogs’ integration.

I have a 4-year-old pit mix (Aria) whose personality has always been calm, cuddly, and low-energy. She’s not had much dog interaction in her life, she tends to get overwhelmed by overly energetic dogs.

About a month and a half ago, we adopted a 9-month-old Cocker Spaniel (Penny) from a shelter. When we first brought her home, she was anxious, shut down, and slept a lot. Early on, she couldn’t even see Aria without barking, lunging, or stiffening up.

We took things slowly: • Started with visual exposure only. • Moved to short parallel walks. • Then controlled yard sessions on leash. • Eventually off-leash yard time, which now goes well. • Added calm garage sessions until both could coexist off-leash there.

Recently, we’ve started bringing them inside together, and that’s where things get trickier. There have been about four fights total since adoption, always short and loud (lots of snapping and growling) but no one’s ever been hurt or latched on. Each incident seems triggered by different stressors:

1. The first two happened early on when Penny was still overwhelmed by her new environment and Aria’s proximity. 2. The last two have been the opposite, Aria starting them. One involved Penny entering the kitchen (a space Aria seems to guard), and the other involved a high-value toy that had been left out. 

We’ve removed the toy trigger and are managing the kitchen carefully, but I’m unsure how to train through this phase versus just managing it.

Both dogs can now: • Spend time together in the yard off-leash with no issues. • Walk side-by-side calmly. • Coexist in the same room for long stretches on leash.

But the household tension, especially around the kitchen, makes me nervous that Aria might be resource guarding space, or that Penny’s nervous energy is keeping her on edge.

Has anyone dealt with something similar where the established dog starts guarding spaces or reactivity resurfaces after initial progress? How do I safely work through this next phase inside the house?

Any advice or perspective from multi-dog households, trainers, or behaviorists would be hugely appreciated. I feel like we’ve come so far, but the home environment still feels fragile.