Thank you for the additional information Agnes....I really appreciate it.
As somone who has spent years rehabilitating dogs with behavioral issues, I want to be completely honest and tell you that although I do feel like we can get Mo to a point where he's more social/friendly/accepting of living life with you, it is going to take a lot of work...but I really do feel like he can be changed.
That being said, the important thing...maybe the MOST important thing to remember is that even though I'm sure you're already invested in him...you are essentiall still a stranger and he is in a strange place, so taking it easy with him for a few weeks, or sometimes even moths, until he adjusts to you and his forever home. I know that you want to have a friendly companion that you don't have to worry about biting you and want to make him a real part of your family, but this can overwhelm these unsocialized little guys pretty fast. Imagine if you were dropped in the middle of Russia...you don't know anyone, you don't know the customs, and you don't speak the language...you'd be a little overwhelmed, no matter how nice everyone was. This is what Mo is dealing with.
I have extra experience with this, because I also own a small breed dog who came to me with a lot of the same behaviors Mo has.With my little guy, the first thing I did was get him a DAP collar to help with his anxiety over the new life he had. These are collars that are impregnated with a man-made version of the dog appeasing pheromone, which is a pheromone that nursing bitches give off to their pups to help them feel calm and secure. It's something that humans can't smell, but it has an amazing effect on dogs with anxiety and other issues. Although you can find them at your vet's office..you can also find them online at places like Amazon.com and Ebay for much cheaper. Just make sure they're DAP brand, as they seem to work better than some other versions. Since it sounds to me like Mo is a reactive dog, this collar will help take off the edge a bit.
I would also get him a regular, hard sided kennel for in the house. I know that you're probably worried about that because we humans view kennels are punishment or somewhere to put the dog when they're not behaving...but to him, a kennel is safe...it mimics a den in the wild so it's where he's going to want to retreat there to feel safe. My little dog was HORRIBLE until I got him a kennel...I put it in an out-of-the-way spot and when he was feeling nervous or scared, he would go there, knowing that he wouldn't be bothered there. At this point, using a kennel will give Mo somewhere to go when he's feeling like this is all too much.
I'd also get him involved in a basic obedience class. Not because you want him to learn to sit and stay (although that's super nice too), but because these kinds of classes will build the bond between you and Mo and will help him build self confidence.
Poms, possibly more than any other breed, love to have a job and to please their owner, and get a great feeling of satisfaction doing one...what you're going to learn in these classes you can take home with you. Telling him to sit might be a fun thing for you, but for him, it's a job. That will help him come out of his shell a bit. It will also allow him to socialize with other people and animals in a safe, neutral environment.
Because this can be a dominance issue, the next thing I'd do is start the Nothing In Life Is Free (NILIF) training program. In my experience, if done consistently, this type of training offers the biggest results for both dogs and owners. With NILIF, your dog will obey your commands without any sort of physical discipline or frustration. The theory is that you raise yourself in the pack heirarchy by controlling all the dogs resources.
More to the point, you're doing to decide when to give your dog his food bowl and when it goes away. You're going to control when you pet your dog, when you offer toys or treats and when you're going to play with the dog.
I like this training because it works on every dog personality. Dogs who are shy get self confidence, dogs who are easily distracted are focused and shy dogs come out of their shell. Dogs like yours, who tend to be a bit pushy will learn patience and manners.
NILIF works like this: we're going to start by essentially ignoring the dog when he demands your attention. No matter how much he tries to get your attention..whether he's pawing at you, or barking at you or even bringing you things. You should absolutely step over him and ignore him as completely as possible. Don't look at him, talk to him or even make eye contact with him. Keep in mind, you're not trying to be mean to him...you're trying to get him to understand that he can't demand you pay attention to him when HE wants it...with consistency, he'll figure out that you're in charge, regardless of what he thinks. As he figures this out...he will become less reactive and less likely to act up for you.
A side note...he may act up more when you first start training him. He's going to really work hard to get you to pay attention to him when he wants you to, but I promise if you don't give in, it'll be worth it in the end.
Everyone in the house is going to have to take part in these new training rules. Make sure they know that from now on, your dog is going to have to earn anything he gets in the house. There will be no treats, no petting, no anything without the right behavior from the dog. When it's time to eat, he's got to sit and wait before you feed him. If you want to give him a little tasty treat, then make sure he earns it by performing some behavior (could be simply sitting). If you're playing with him, only continue as long as you want, then pick the toy up, put it out of reach and walk way. Consistency is the key!!
The botXXXXX XXXXXne here is that NILIF is going to establish a very clear pack hierarchy in your house with you and any other human in the house in the top spot. It's done without cruelty or physical punishment, but will get the point across. In time, your dog is going to stop doing anything that you wouldn't agree with, and that includes trying to nip anyone in the house.
I also think, he should be walked with a Gentle Leader. I guarantee that he will eventually get used to it, and it will make him easier to control in the walking. The general rule of thumb is that if you control the head, you control the dog, so putting the leader on and leaving it on during the walk, regardless of how much of a temper tantrum he throws is key (And I'm sure in the beginning he will). When he starts pawing at the Leader, simply give the leash a tug, say "Leave it!" and continue walking, even if it means you have to pull him along for a minute or so. Physics tells us that the dog can't paw at his face and walk at the same time...if you keep him moving forward, he'll have no choice but to leave the Gentle Leader alone.
Secondly, you might want to consider clicker training this dog. This involves buying a 'clicker' which has a little metal plate in it. You press the plate and the thing clicks, then you give the dog a treat immediately. Soon, the dog learns that the click brings a treat and will usually drop whatever they're doing, and will even will help him focus on you during the walk. http://www.clickertrainusa.com/clicker-training-videos.htm .
As I said...I think that Mo is going to be a lot of work...and I wouldn't expect to see immediate results...but I have zero doubt that with some time and consistency, we can get this done.
I hope this helps.