What kind of dog should we get?

My girlfriend and I want to get (another) dog. We already have two small dogs – a boston terrier/chihuahua mix and a pomeranian.

My criteria:
MUST be friendly with our small dogs and preferably most small animals.
Must be trainable: obedient and fairly quiet indoors, not aggressive, etc.

Her criteria:
Bigger: 30 – 50 lbs
Must make a good running companion
Must be a fairly good guard dog/protector (or at least look capable of it!)

So we want a medium/large dog who will be friendly and calm with our other pets and visitors, but who will love to go on runs with my girlfriend and would protect us/our home if necessary.

She likes german shepherds, siberian huskies, and shiba inus. We don’t need a purebreed – we will be adopting – but we want to know which general direction to look. Would any of these breeds (perhaps mixed with something else…?) work for us?

There will be someone home with the dogs most of the time. We will have a small/medium fenced yard.

Thanks!