I've always had multiple mutts together. Some take to it well and others there's no way in hell. I've found in 20 years of doing this that it only takes one real incident for some dogs to just simply not ever want to be around the others again. More times than not I have found that certain mutts will tolerate some and not tolerate others. Like they pick and choose whom they will be ok with and whom they simply will not allow in their presence. JM2c....
I have my last 2 girls (9 & 5 years old mother and daughter that have been together from the day the daughter was born yet neither will tolerate other dogs) The daughter will go after her own pups at around 5 to 6 months old. These 2 will be the last ones I will even attempt this with as it only takes a split second for an accident to happen. Usually over something stupid like who barked at the door first...