Currently I am living at my boyfriend's house, and recently his sister and her husband have moved back in. Up until then it was just my boyfriend's mother's dog (female staffy, named Sooty) and my dog (male jack russell x whippet, named Leo). These two got along fine, played together, etc. Now that the sister and husband have moved in we now have two puppies living with us also. One is a female kelpie x border collie x something big (named Lily), and the other is a male bull arab (named Obi). All four dogs are desexed.
Things have definitely changed since they've moved in. They're puppies still (one's a year old and the other a little older then that), so they love to jump on the other dogs, chew on everyone and every thing, etc. All the dogs (except Obi)) are very toy driven and as soon as a ball or rope is found they try to play tub of war. We have found, however, that allowing them to play usually ends with Sooty getting jumped on and someone causing a fight. Since my dog is the smallest he seems to cop the worst of her wrath. Both these new dogs are twice her weight, so we think she might even have a sore back because of it.
However lately we find Sooty gets very protective of her mum (boyfriend's mother) and she will try to "beat up" Leo if he so much as even looks at her. We find this only happens if she is in a "cranky mood" (ie, she's uncomfortable because of her back) or if either of them get attention from "mum". Needless to say, he is a very fast runner and seems to avoid confrontation with her.
I am hoping once the two other dogs aren't in constant contact with our dogs (sister and husband are mostly through building a "removable" house in the paddock) things will calm down. In the meantime we try not to allow all four dogs playing together and try not to be overly generous with attention. If there is a fight, they're separated and ignored, but mostly we've gotten pretty good at picking up her signs early and stopping it before it begins. Other then this, all four dogs are happy and healthy, so let's hope this is just temporary.