OK first thing, an older cat is NOT going to appreciate live with a kitten -- all the baby energy! And the kitten is going to want a play mate, so you are setting them both up for failure.

If you must have a kitten, I think the oldest the 2d cat could be is about 4 years.

Shelters and rescues have plenty of older kitties who would LOVE to have a home. You could get 2 between ages 5 and 9 for example, and that would be great.

When you first introduce 2 cats, you don't leave them alone together. Keep them in separate rooms, one can roam the house the other in a room, then switch off who is where. Put towels in their beds, switch the towels daily so they get the scent of the other cat. When you are home and have time, let both out. Have a water squirt bottle close by to break up any fights. Expect some hissing, even a swat if one gets too close before the other is ready. It can take 5 days to 3 weeks to have them living in harmony.

Keep your goals reasonable: you can expect them to live in peace. You can not make them best friends, who play together and sleep together, that they must decide for themselves.