TimeLord Prime- Maybe you could have Seven as a cat next and we could see how Ace reacts to it.
Never really written for Seven and Ace before, but...
A small figure climbed over the chain-link fence in the darkness and landed on the ground with a heavy thump.
"Ace!"
She swung her rucksack around to her front and unzipped it. "You all right, Professor?"
A tabby manx stuck its head out. The Doctor had yet to tell Ace how, exactly, this transformation had happened, or even where he'd found a cat-sized panama hat. "I thought I told you we weren't going to need your nitro-9 this time," he said, gesturing to the grey cans beneath him.
"You did."
"And I also thought I told you to be careful if you're going to be carrying high explosives and living creatures in the same bag?"
"Yeah."
"Just checking." He turned his attention to the building in front of them while Ace ducked behind a row of bushes. "Both doors are solid steel, but there's a side entrance that's heavily guarded. There's also an open window on the ground floor-"
"And I'm not to do anything particularly distracting to throw off the guards, especially not with the nitro-9 I didn't bring?"
"Exactly," he replied, tapping her nose with a paw. The Doctor hopped down from her shoulders and briskly walked along the fenceline. "Thirty seconds."
Less than ten minutes later, both girl and cat came tearing out of the building as smoke poured out of the windows. The Doctor ran along the top of a brick wall for a few metres before jumping onto Ace's rucksack, halfway balanced on her shoulders. "Ace!"
"It's all right, Professor! Just a miscalculation!"
"Miscalculation?!" Behind them, the building blew up sky high. Ace dove around a corner and concealed herself and the Doctor within a deep doorway, narrowly avoiding falling debris.
"Well, it put an end to the Cybermen, didn't it?"
"Oh, Ace..."