Looks like size and age matter.
It's Tabs' battle this time...with the black kitten.
Tabs was growling at the sidegate...
Just as I feared, the black kitten had walked in (no need to squeeze) through the bars of the grille.
Tabs was on the aggressive, with tail all fluffed up, defending her position as the "baby" (youngest) of the clan.
Bunny was just behind Tabs, watching, not reacting at all. Just watching....I could almost hear Bunny saying, "Now, this is your fight, this Blackie is too young for me. I don't bully young 'uns. You take on him."
Don't react, and there will be no stress. But that's not the way of the cats. For that matter, some humans also don't understand this simple wisdom!
Indy was next in line, behind Bunny. being the reinforcement should Tabs need help. After all, age hierarchy-wise, Indy is the next youngest. His position may be threatened too...
Updates at 10.30pm:
Blackie came in and started using the scratching post.
Tabs is no longer on the aggressive, but is more of being curious, I think.
I had to put Blackie outside Bunny's Place because I don't know what disease he may be carrying. He ought to be safe in the garden.
He's already eaten and has drunk water too.
Updates at 11.40pm: My husband wire-netted the grille. Blackie and Tabs watched as we did it, one of each side of the gate!