The important thing is to let them get on with it, remove all resources like toys, half eaten bones etc then be there just in case ignore the established dog saying stuff like what is it? Is it staying? Thats my toy? do not reinforce any behaviour just ignore. Let the pup sniff about and find her/his feet the exits in to the garden the position of the beds the food bowls etc
At what age is best to get a new addition?? Ideally?
it depends on the resident dog and his/hers state of physical health and mind and how well trained or not that dog is and what time you have - dogs communicate to each other faster then lightening - bad stuff gets taught as well as the good - it really does depend on the house system in place