any dog behavior experts advise wanted

not sure if this is the right place to ask ,but im desperate,i recently lost my dog and best friend of 16 years, libby ,and thought as she was so good to me and for me ,id try to give something back by adopting a stray from the city dogs home ,i ,or should i say she found me ,a lovely young cross breed ***** of around a year old found on the streets and very neglected and skinny ,anyway we introduced her to our other two dogs ,a yorkie ***** almost 14 years and a lasa dog 5 years ,on 2 separate occasions a couple of days apart at the dogs home ,no problems , brought her home all was well , the yorkie is a bit of a grouch now and then and she growled at maggie ,the new dog and maggie cowered ,i thought that showed she was deferring to the older dog ,good sign ? ,i thought so ,then after a hour i was sitting down on the sofa ,maggie sat next to me ,pepper ,the yorkie came towards us ,now i didn't see this as i was behind maggie ,but my daughter said maggie bared her teeth at the yorkie ,but the yorkie kept coming ,probably didn't see or hear the warning ,next thing shes got the yorkie by the neck ,i picked her up and my daughter got the yorkie ,i took maggs out side and see turned into terminator ,she literally changed ,she went into guard dog mode ,patrolling the garden and barking ,very cold and calculated ,not crazy excited ,she went from a big daft puppy to a all business guard dog ,we kept the dogs apart as she was barking like crazy when she saw them ,kept her overnight and had to return her yesterday,heart breaking to return her to that little pen after having her home with us ,anyone know of any ways to train her or socialize her ,i really want this to work out for her ,shes a fantastic dog ,she was great with my family ,and settled well with me and my wife and kids and the random sounds of our house and street ,shes a sweet nature ,let me take her food and a bone she was chewing off her ,no aggression or problem at all,just seems to be other dogs ,did i do something wrong in the way i introduced her into the home ,and is there some way i can fix this , im clutching at straws here but anyone been in this situation ,any way i can help these dogs get along , i know she must be tramatised being in the home ,30 odd dogs all barking and upset ,cant be good for you ,and she showed signs of settling after only a short time of being with us ,she just went in to protective mode ,,when i took her back yesterday she barked when anyone approached us but accepted them when i reassured her ,please any help ?
here she is when she chose me at the dogs home
heres maggs pepper and piggy ,at the dogs home

and here she is about 2 hours after getting to our home ,sleeping on me

thanks for the reply no ,not the vet ,i spoke with the staff of the dogs home ,some were "well shes a young dog whos of mixed breeds, so who knows what her temperament is going to be like in the future " to well you could try a different dog ,or muzzle maggs ,the new dog ,for a while ,so she cant hurt the others until she gets used to them ,the dog warden said walking all three dogs at the same time ,away from the home would be the best way to go about it ,that would reinforce the "pack"mentality in all 3 dogs ,bonding them into a unit ,its just pepper the yorkie is almost 14 and im not sure shes up to or needs all this upheaval at her time of life ,but dogs are adaptable or are they not ?

I've worked professionally with dogs for about 12 years now. It sounds to me that Maggie was guarding you from Pepper. Without testing the dogs myself and observing them, I couldn't tell you anything difinitive, but Ween is right in saying that multiple dogs do need to work out a hierarchy, and that hierarchy is constantly changing as well. While I think it's great to want to help a needy animal, you're main responsibility and concern should be on the ones you already have. Pepper is old and probably deserves to live out the rest of her life without having to be pushed out of her place in a newly evolving hierarchy.
Maggie seems like a very sweet, cute dog, and surely deserving of a good home, but if she has it in her to grab Pepper by the neck, that's not a good sign and not safe for Pepper. If you still feel the need to rescue another dog, I would suggest one smaller, that couldn't as easily injure you're other two.

I was waiting for someone who knew more than I do about dogs to reply, but it's just Ween saying pretty much what the problem almost certainly is - somehow your interactions with Maggie didn't reflect the natural pecking order that you had established with the other two. When you say that the first incident occurred when Maggie was sitting beside you on the sofa, that to me rings alarm bells. The sofa is where the leader of the pack (you) sits, so proximity to you is prized real estate. Yorkie comes in and there's the newcomer leapfrogging the existing pecking order by sitting next to you. And naturally Maggie wants to let Yorkie know all about the new pecking order. The first encounters between the dogs were on neutral territory - I think the issues began at home, on your two other dogs' home turf.

I'm sorry it didn't work out with Maggie, despite your best intentions. And I'm sorry for your loss of Libby. Our little pooch is in his twilight years, it'll be a sad day when we have to say goodbye to him.

Don't let this experience put you off pound puppies. My sister has a pound puppy, 5 years old now since bringing him home as a one year old. The most beautifully natured dog you could meet. Dogs are adaptable, but as the pack leader you need to identify the group dynamics and instil some consistent discipline and leadership. Good luck!

Edit: written before I read DrOdd's authorative post, which also sounds spot on to me

she looks lovely. the pics surely shows it. i hope it works out for both of you. my heart is sinking...

thanks for taking the time to help me out with this you guys ,i appreciate your input and good wishes ,yeah your all dead on my first concern is the other two dogs my wifes and my daughters ,my daughter is 19 by the way ,thats why we returned maggie to the dogs home ,firstly to protect those little guys and give maggie a shot at a home with out our complication of 2 other dogs ,my hope is she can find a loving home ,and i can at least help her by telling any prospective owner of her many positive traits,travels well ,loving nature ,house trained ,doesn't chew expensive vintage super hero toys and high end collectables lol ,etc,and research to see if there is a way to reintroduce her into my home and socialize her with my other two dogs ,ive had some great suggestions and encouraging news ,that dog whisper guy is magic,and his methods make so much sense and seem easy ish to implement,and im sure that maggies problem can be straightened out by using his methods ,so for now maggies at the dogs home and i hope she gets lucky and finds a good and loving home but if not my family want us all to give her another chance with us armed with a better knowledge of what went wrong last time and how to help her to accept our other dogs this time ,again thank you all for your help and kindness and ill keep you posted as to how it works out,cheers chris

That's a great article, alittle ironically timed. The " nothing in life is free" technique is something we employ with our clients, and I personally use the seniority type attention order with my 4 dogs. Both combined are a good start to dealing with multiple dogs.