That’s what happened with Mr. Boots and Scout. There was a heated confrontation, then they were inseparable.
Also, get second litter box and park it along side of the first, and make sure that everyone has their own food bowl for soft food for a while.
I’m not big on soft food, but for the short term let this be one of those things where the dry food is community, but everyone has their own bowl, in different locations in the kitchen at breakfast. They will eat separately, find their own spot for a bath and then sleep off breakfast.
BTW the old man will love the “high fat” dry kitten food for a while but don’t let him eat too much of it ;-)