The current oldest in my house is a siamese-type cat, Yoshi. He is about 11 yrs now. He has 'endured' the additions of 5 week old basset Holi [6 years ago], rescued 6 mth fluffy queen cat Precious [first we changed her name to Gabby then spayed, 5 years ago], goofy 5 yr GD with no physical awareness
[1 year ago], found 5 week kitten terror [6 months ago], and an adult beagle we found a couple months ago who has NO cat experience [none of the cat's are declawed and will smack a too inquisitive nose when needed].
I'm telling you this because all the cats have handled the successive new additions well. If Yoshi doesn't approve, he will ignore me and the newbie. He was an only child for a long
time but has adjusted very well. I know there are plenty of cats that don't like change but have found that if things remain as normal as possible and they are allowed their own space to escape to, they will work it out shortly.
BTW, I LOVE Marco's name! Very inventive
I look forward to hearing how things went. Perhaps pictures