I have four dogs and I am often asked how they all get along. They are wonderful together. I believe it's because I have been very cautious about the types of personalities I put together and I research breeds very carefully before bringing a new member into the pack.
Simone is a female collie who is about five years younger than Mattie. She has never tried to exert any authority over Mattie but seems perfectly content to be second.
Eddie and Lucy are Jack Russells, though Lucy is most likely part Basset. Eddie has endeared himself to everyone else by grooming the others. Lucy is the youngest and she is always ready to play. Because they are much smaller than Mattie and Simone, antics that wouldn't be allowed with larger dogs are tolerated by the two oldest - such as an insistence to play that can be relentless.
Shown in this picture are Eddie and Simone.