Cats/Older Kitten Adjustment
We adopted a 7mo old female companion for our 6mo old male kitten, who we've had since he was 10-12 wks old. At first the 7mo old female hissed at the younger 6mo old kitten but for the most part it stopped after 3-4 days. The 7 mo old female kitten appears significantly larger than the 6mo old, so I'm wondering if the truth may have been stretched to get the kitten adopted. They also were adamant that she gets along w/ other cats. They do play and chase each other but the younger 6mo old male is far more palyful and has continually extended the olive branch. He seems somewhat put-off and more impatient now. The 7mo old female tends to want to be a part of and dominate most play time. She's a sweet kitty but I'm concerned if our 6mo old male will again be a lovebug & affectionate kitty. He used to sleep in my husbands lap all the time but hasn't since we've brought the 7mo old home. It's been 11 days. We're wondering if a younger kitten would have been a better choice. We can return the older female kitty and I'm sure she'll find a good home b/c she really is sweet good cat. Thoughts?
Eleven days is too soon to make any calls, in my opinion. Cats adjust to changes slowly, and beyond a few months of age, it typically takes about two weeks before you can leave them unsupervised in a room together. I think you're actually ahead of the game if they're interacting fairly well.
I think your male's change in behavior is likely temporary as he tries to learn exactly what his limitations are around the new kitty. Cats have personalities that range about as widely as humans, and it sounds like you have an extraverted female vs. a slightly more reserved male. But that usually doesn't mean one kitty will be bullied - it just takes them some time to figure out how to interact.
Your male should have some extra encouragement to take part in the old behaviors that the family enjoyed with him by using treats. Your female will probably partake as well, and this is fine, since sharing treat time together tends to solidify bonds. If the female becomes too pushy at times, see if you can put her in a separate room for a while so the male can get his lap time in. As he starts to feel more at ease around her, I think he'll feel braver about taking his spot back.
It's possible one of the kittens' ages isn't quite accurate, but they all develop at individual rates and will end up at different sizes as adults. It could end up that she's a large kitty, and he may be petite. Tooth development is the best gauge of age, and your vet can give you an estimate of her true age, but this has wiggle room of a month or two, as well. In the end, a tiny cat can still end up dominant over a large one, and I think you still have a few months to see who will win out as top cat in this battle! Most of the time, it isn't really settled until adulthood (and it is delicate after that).
So, the bottom line is that in my opinion, it sounds like you're actually progressing well, and I think given another month, things will be closer to normal. But if you are very nervous about waiting things out for another month or two, you may want to return the new kitty. I would probably hesitate to introduce another kitten. It's possible your male will react the same way to any new pet (a temporary change in attention seeking behavior can be a normal stress response).
Best of luck!