Be prepared for an extended introduction period. Introducing kittens into a household is almost always much easier than introducing adult cats.
I introduced a 3 year old F2 to our house with a 4 year old F6 and a year-old domestic medium hair mutt of a cat. It's taken almost a year and everyone is on mostly good terms most of the time. I can leave them all alone together without worries. And it's still improving.
Some tips: As Pam says use as much of a quarantine period as required. Make sure he bonds with the two of you during this period--which may mean the two of you taking turns sleeping with the cats separately.
Start feeding them somewhere they can see each other, but not get to each other. I had sliding glass doors that I could feed them on opposite sides of.
Switch spaces once all health concerns are past, so that they get to know each other's scent, explore their space, and use each other's litter boxes.
I also borrowed a small dog crate that I could use to allow the cats to see and smell and interact, while not being a danger to each other.
Prepare for them being together by giving them spaces to get away--high structures, shelves, trees, book cases, and high paths so they don't get or feel trapped.
Playing with them together can help, but can also lead to jealousy issues. Our mutt, Jax, hogs most string toys and our F6, Oni, gets frustrated by this and sulks. I also caused a couple of collisions and swats while playing with all of them before they were comfortable getting that close.
At some point you may need to let them work things out themselves. Cats can play amazingly hard, and what might look like a fight might be play. Watch for the ears to come back up rapidly and for them to respect when one of them yelps or otherwise indicated enough is enough. Oni needed to figure out that Ziggy wasn't going to kill her, and our intervening every time she screamed wasn't helping.
Oni noticeably pulled away from me for months. I think she blamed me for bringing Ziggy. She is finally now snuggling on the couch with us even with Ziggy there. It's still very rare that she joins us in bed, especially if Ziggy is there already.
I've had some success with clicker training them individually and then together. I'm just not consistent enough with it to see its full potential.
Best wishes! I'm glad you're trying to give this older cat a good home. All three of mine are rescues and our home is far richer for having them here.
