I just wanted to share my experience although not to change your mind OP as it sounds as though you know what is best for your dog.
We've had Bella our Romanian rescue for just over 8 years - we got her when she was 6 months. She'd come from an environment where, as @yorkshireteapot9 said, she'd had to fight for food and had been attacked by other dogs. She was very reactive to other dogs on walks and would resource guard.
8 years of work and proactively creating positive experiences she is now so much better and we've had friends dogs over to stay (ranging from a weekend to 2 weeks). When they've stayed we've followed some clear rules:
• Always go for a walk together before the guest dog enters the house so they both enter together.
• Remove anything which could be guarded - Bella would suddenly take an interest in toys which had been ignored for years!
• Always feed in separate rooms.
• Manage seating arrangements on sofa
• Making sure when Bella goes to her bed no dog approaches her.
• Not allowing more playful guests to jump on / harass her to avoid any potential escalation.
• Not letting dogs approach her when she is in her den (under our bed).
• Do not leave them alone together (depending on guest).
After a few days Bella would be much happier with the guest dog and even play with them (she has to really trust dogs to play).
We decided to get Sprout (4 month rescue) who arrived at the end of January. Beforehand we got advice from a clinical behaviourist who had previously worked with Bella which covered introductions and the first few days. The behaviourist then visited a couple of days after Sprout's arrival to give us any more advice on managing the environment etc. Our goal was for indifference from Bella towards Sprout.
Bella did not welcome Sprout with open arms and steadfastly ignored her for the first couple of weeks - we continued following the above rules and also spent lots of time with Bella one-on-one (arranged a friend to puppy sit while we both took Bella out etc).
Initially when they were in the same room together Sprout would be in her play-pen and Bella became more comfortable in her presence.
When Sprout progressed to being out of her pen we would supervise at all times - the important thing was that Bella trusted us to manage the situation so there was never a need for her to escalate her 'go away' behaviour.
7 weeks later they are absolutely wonderful together! I've never seen Bella be so playful - she is the one who is regularly instigating it and they play multiple times a day. Bella is clearly very comfortable with Sprout as she will happily lie in her back during playtime, let herself be chased etc.
They are still fed in separate rooms and Sprout isn't allowed to pester Bella etc.
The careful introductions and ongoing supervision has definitely been key - plus, the 8 years of work beforehand 🤣 although I should say that if we hadn't got to a stage where Bella was comfortable with guest dogs we wouldn't have attempted to get a puppy!