Adopting a rescue cat when have a cat already good idea or not?

[Sunflower6]

I have a lovely boy cat aged 7 who is a really loving family cat. We did have a girl cat age 13 who was a lot more independent but she has been missing 3 months now. I am thinking of adopting a younger cat as I miss having two cAts. I do have in mind that my girl could possibly return but she's never done this before. I am just worried if adopted cat and existing boy didn't get on

We did this twice. The second cat died within a couple of years so we then got a kitten. The first cat tolerated them. They were never friends never really fought, the house was large enough for both of them. The third cat also died very suddenly and very young. We didn't get another as we felt it wasn't fair on first cat.

I heard a vet speaking on this topic the other day.
He said it wasn't a great idea. Cats are solitary animals. It may upset your own cat.