Cat integration tips?

We have two usually happy Siamese, Tycho and Sophia. They're 26 months old. We just acquired two ... large shaggy lap monsters, possibly with Maine Coon ancestry. Archimedes (uses his tail as a lever) and Nikola (dark and brooding, like Tesla) are about 14 months old.

Archie and Nik are currently isolated in a bedroom with water, food, litter box and frequent company, including sleeping boys.

Tycho and Sophia haven't seen them, but are very nervous. Tycho has been hissing at me, then running away.

Anyone have reliable advice on procedures for feline harmony?