I'm asking everywhere!
We adopted two litter-mates in Dec 2015, Walnut and Cashew. They loved, loved, loved each other. Sadly and a story I won't go into, Cashew died October 21, 2019 of intestinal lymphoma.

I'm still reeling from it, but that's besides the point.
Walnut, after a bit, seems to have adjusted quite well. He's a happy guy, eating well, still playful, etc. He is having an ultrasound done because he has hairballs, but mostly for our peace of mind, but other than that everything is perfect with him. He doesn't run up to people and love on them like Cashew did, but he's certainly not scared. The vet says he is the most laid back cat they've seen in a long time.
We want to eventually bring another cat to our home. Walnut will be 5 in March and I have some questions:
1. Is 5 too old to bring in a new cat?
2. Female, male or doesn't matter?
3. Because Walnut was so close with Cashew, will he reject all other cats?
4. We want to adopt a young cat - 6-9 months. Yes, no?
5. Would 2 kittens be better to adopt so they have each other if Walnut doesn't want to play?
I want to do right by Walnut. He's what's important.
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Tell me your advice please and thank you!