Our OP has not posted in awhile. I can only hope she reconsiders a few things.
Here are my beliefs:
Some large breeds are more suited to our Yorkie breed than others.
But all larger dogs to Yorkies and that is a whole lot, need to be trained, and the Yorkie protected until the new dog is obedience trained. Think about any dog that weighs 50% or more than your adult Yorkie. Damage can more easily be done inadvertently by the heavier dog.
Rules of "engagement"
1) Be prepared to crate and rotate until and unto such time as your new dog is fully trained for the basic obedience commands. This means only under the most limited circumstances is the big and the little allowed in the room together even WITH supervision, and never without supervision.
2) Know your dogs. That means what is their prey drive like. When they are puppies you will have only a inkling of their prey/toy/defense drive.
3) Be prepared to exercise and train separately, and one day hopefully together.
4) Train your new dog to never interfer with their pack mates eating. That means you stay present in the room in which you feed the dogs, and physically block if necessary one dog from going to the others bowl.
5) Likewise always discourage toy sharing and playing tug of war between two dogs of disparate size.
6) Never play fetch with one toy and two dogs of disparate size. That is a disaster waiting to happen. And be very cautious of two dogs of even the same size. I would not do it on wet grass or slippery ground, too much room for twist/rotational injuries.
Some-one mentioned that this seems like a stressful situation to live under. Perhaps it is for you, think about it. It certainly requires forethought and planning and a commitment on your part for likely 2yrs or maybe for the lifetime of your dogs, just depending on how everything works out. Are you ready for that?
This year I hope to add a female BRT puppy into our pack. At 8wks old she will likely weigh more than 3x Razzle's weight. For the first couple of months or more depending on how our large Male responds to a pesky puppy, she will sleep at night in a crate, as will Razzle continue to do so.
Razzle and this puppy will have very very supervised interactions, and maybe for the first month or so with both on leads. The puppy when very young will have her gated Xpen to be part of our pack, and will go on walks - separately, then with Magic, then with Razzle. She will start obedience training the moment she comes home, and then formally as soon as last shots are given. Hopefully she will be allowed out in the yard with Magic, but never with Razzle unleashed. That will only come when she is at CD level of obedience and only if her prey drive is not high. If her prey drive is high, that may mean never with Razzle unleashed - free in our backyard or outside.
OUr belated Zoey with a very high prey drive, only got to being allowed free with Razzle in the house, and NEVER outside off lead together. Also we train no chasing of toys in the house for obvious large dog reasons. We do play hide and seek the toys, and we will practice heel on lead and off lead, and come down sit stay, touch, drop, leave it, etc.