my experience is that they are going to along or there not. ive brought more than a few second dogs in the mix and only one didnt like the others, but she was also 4 years old. no fighting, she just didnt like the others. like you im always concerned each time, and should be. i had a shitzu and brought a pug, good friends, in fact ended up being best friends, then another pug to my pug, again friends, then a french bulldog to my pug, now that i worried about for months, seeing as the frenchie is going to outweigh the pug by a bit, but all is well and they are becoming good friends, playing and sleeping together. like Fosta Mama said = it depends on your dog. Does your dog like other dogs? my dogs have always had another dog to keep them company.