I would use the “wet flannel” technique....
Take a damp flannel and rub the sides of the face, flanks and ears of one dog, then using the same flannel do the same to the other dog. Repeat 2-3 times before letting them meet on neutral territory. Leave their leads on just in case but don’t hold on to the lead - so many times we read about dogs that are nervous being worse on lead. Don’t assume if they get on initially they will always do so. We have 5 bitches and there’s a regular ding-ding in the lead up to a season. It settles again after and is “hand bags at dawn” but fairly shocking to an observer or the first time it occurs.
Wash all Minas bedding, toys and fleeces/coats to minimise her scent in the house. Bring the new girl in with her scent so she can get established quickly. A Feliway plug-in that emits pheromones may help settle both girls.
Prior to Coral arriving Pearl slept upstairs. One really good piece of advice given to me was to treat all dogs fairly and equally. I had 4 days notice that Coral was arriving and chose to put Pearl in the kitchen as soon as I knew the other dog was mine (Rescue). Interestingly the anxious dog that sometimes guarded preferred to be in the kitchen as it was less responsibility than having the whole house to roam in. Coral was crated from the first day (age 9 weeks) so I knew she would be safe.
All 5 of ours sleep downstairs. 4 are crated and ask to go to bed in their crates. 1 sleeps where she chooses - she slept in the bed when I first met my now husband
We used the same techniques and approach when we rehomed Purdey, 3 years ago. She came into an established home with three other dogs and Kukri (now age 23 weeks) slotted in easily as number 5
You will need to be patient with the new girl, it can take six weeks or longer for a rehomed dog to settle into their new environment. All the sounds, smells, commands, voices, routines, food, even the level of light and the direction she will turn out of the front door will be new to her.
I would suggest training them separately to start with to give quality time to both and if Mina becomes a problem with guarding seek help from a behaviourist.
Hope it all works out for you,
Jayne