My vet says that the younger the kitty, the more likely it is to be accepted by an older cat.
I used to take in whatever cats God sent our way, put them together, and say, "Now, you all had better get along!" And, for the most part, they did! (I still do that, to a lesser extent.) The one major problem was the time when we took in an older male cat, and our oldest female cat got territorial.
On the other hand, that SAME female cat (now 17 years old) was found as a four-week-old kitten, too young to be separated from her mama. Our three-year-old cat (at that time) absolutely welcomed her in, took care of her, and played with her - they were best friends for years, until he passed away.
If you adopt through a rescue agency, you'll find that some agencies evaluate their foster cats and can tell you whether they'll probably fit well into a home with another cat already there. You can pick their brains about your other concerns, too.