I have a 10+ year old mix breed and an 8 year old mini dachshund. Last year, my daughter brought home an 8 week old standard poodle puppy. The acquisition was such a sucess with the other dogs that my husband and I added a new standard poodle puppy this fall!!
My pack gets along great and the older dogs still have the same position in that pack - they didn't get ignored or put aside for the new pups, in fact, we included them more than ever so the poodles could learn how to be great family members from them!
As Trillium said - keep it fair and you should have no problems! Good luck!