I adopted a new kitten approximately 4 months ago; Mister Black. He was an older kitten, and has had persistent diarrhea since I brought him home. I don't need advice in this regard - all options are being explored with a vet and I've spent many hours trawling the web for potential solutions. The vet's opinion is that he's all-around healthy, minus the diarrhea.
He's got definite crackhead energy, and receives plenty of play to wear him out to the point of panting. Still, he will occasionally go directly from cuddling to biting or clawing my legs. I'm starting to suspect his original home may have played with him using their hands. I respond with yelps, removing myself from the room or re-directing to a toy, but it doesn't have any lasting effect.
I already had another cat, Mister White, who's frankly spoiled me with his good manners and gentle spirit. I've had him since he was a tiny kitten just ready to leave mama, and he's a year older than Black. The two of them can hang out supervised, but White distrusts Black after he's tried to roughhouse one too many times. They were introduced slowly with scent swapping. Their playstyles are completely different, so I haven't managed to find a way to play with them together (Black enjoys adrenaline and muscle play, White enjoys stalking and chases with no contact). They can eat together just fine, although Black has to be actively discouraged from eating White's food.
Black is being castrated next month, which I'm hoping will help alleviate some of his issues (It's taken this long due to waiting for his medical issues to resolve). But I'm concerned that it might not - in which case, where do I go from here?
tl;dr: kitten is Satan and old cat is Jesus, what do?
ETA: It's worth adding that I have 18 years of experience with cat ownership with multiple different cats, and I haven't experienced anything like this before.
No worries! Food sensitivities can make this harder, but not impossible. What we're looking for is a way to reward Mr. Black for disengaging from Mr. White - this can be treats, play, physical affection, really anything that Mr. Black enjoys (besides being a dick to Mr. White). The set up:
Mr White exists, Mr Black is doing the behavior before it escalates to the problem behavior. (If this change happens too quickly to interrupt, pick an intervention point based on their behavior pattern, just earlier in the cycle.)
You call/get attention from Mr Black, and reward this change in focus with some play or scritches or whathaveyou.
Mr Black is allowed to go do his own thing again. Repeat as necessary.
This intervention helps to teach Mr. Black that at a certain point in his excitement building, it's far better to go get something awesome from you at that point, because he'll get to go do whatever after he checks in with you. It will take patience and repetition, but it is doable (we had to train a pair of older brothers when one's health started to decline). The act of checking in with you allows Mr White to relocate if they want to, and "resets" the excitement level Mr Black is experiencing.