my older dog was 6 years old when i got my younger dog as a puppy. tiger isn't good with other dogs so i did wonder how she would react but i just sat on the floor between them & let them say hello whilst watching carefully & ready to rescue poppy if needed. tiger tried to ignore poppy for awhile then seemed ok & then when poppy batting her on the head with a paw tiger pinned her down by the throat but i'd been watching tiger carefully for the signs she gives when she's about to attack & their were none. when tiger pinned poppy down it was her telling poppy off not wanting to kill her. she let poppy up straight away & poppy was a bit more respectful then. now 6 years later they are best friends most of the time. occasionally tiger will start a fight the last being beginning of the year, one before that was at least a year before so i can deal with very infrequent problems like that especially seen as poppy will defend herself but back away at the same time so neither of them have ever got seriously hurt