This is why I like ferrets :).
They rough-house all the time, they love it, but the separation between "combat mode" and "play mode" is absolutely clear. And if they're playing with another animal that goes too far, cat or dog or whatever, they have a totally in-control "warning nip mode" that delivers pain but no damage, and then they stare at the offender...and the threat is REAL obvious. Full combat mode is something I only saw once, when one got attacked by a much bigger dog...and wow did that dog get screwed up. A berzerk weasel clinging to your neck by the teeth while ripping with all four sets of claws is one hell of an equalizer.
That's why they were so friendly with strangers, critters, whatever. No fear. They may have been small, but "unarmed"...not hardly.
Smart little boogers. Used a litterbox. No dominance struggles, then again Mikey the bigger one was all of 3 pounds :).
They also have a really warped sense of what "play" is. They love to be flung through the air bouncing on a sheet held at all four corners :). They do this happy wiggle as they get up near the ceiling. And yes, I really mean they like it - my brother and I could grab a sheet, lay it on the rug while we held the corners, and the guys would just come scrambling over and race each other to the middle of the sheet. They knew we'd only do one at a time, they each wanted to go first!
God I miss the skinnykitties :(. Stolen in '97. Sigh.