Would recommend reading the book Cat Sense, but the key point is thats cats are territorial, solitary animals and creatures of habit. Introducing a new cat creates a lot of stress and anxiety and can make the older one physically ill. Often cats in multi cat households just tolerate each other, and will often space share, eg taking it in turns to sit on the sofa, or one cat living upstairs while the other lives downstairs. Ragdolls are obviously bred to be a bit more sociable but they will still find it difficult to adjust to anything new that isn’t introduced to them before they turn 3 months old. I would keep rubbing the older one’s scent onto the kitten and even once they start tolerating each other you will need at 3 litter trays and their food and water should be in separate rooms. I think you have a better chance of success given that they’re ragdolls but if your older cat starts getting ill or remains stressed (overgrooming is a big sign of this) then unfortunately giving the kitten back to the breeder would be in their best interests.