here is what i advise -- being that i am a big dog person. from what i know about mastiffs they are pretty laid back even kind of oafy in a good way. you seem apprehensive about the mastiff and how he might interact with your current dogs. please do some research before you actually commit to a mastiff. get some books, talk to breeders, go to dog shows and ask lots of questions. arrange for your dogs to have a playdate with a mastiff if you can -- gauge your own and their responses. ultimately you have to be comfortable with the dog or the situation will not work. the process may take a while so please put forth the effort. if you find you cannot handle the idea of introducing a big dog talk it over with your husband maybe he would compromise with a med sized dog more suitable to your comfort level. if you get the dog and realize you can't handle it then it is really too late! if you decide you can handle the mastiff you need to go to a reputable breeder -- more research. are your females spayed? they probably should be and so should the mastiff. if you would like any assistance in choosing books to read or finding dog shows near you feel free to pm me. hope this helped.