I think you have to do what you feel is right for you.
Much of it depends on how much time/work you can put in with the dogs, do you have the time to walk them separately if necessary?
Adding a third dog whatever age/sex will alter the 'family' dynamics - it is just whether it is for bad or good!
Yes a young adult rescue could teach Mishka some bad habits -
or her good habits could rub off on the rescue.
My first bitch was around 8 months old when I started looking for a companion for her, by the time I had arranged to buy my BSD she was a year old. The day after I brought my 16 week old puppy BSD into the house, I also rescued my GSD's litter sister! So from being an only dog for the first year of her life she suddenly had two new companions! A pup and her year old sister! I also worked full time, so I really had my hands full

. Everything worked out though, because Kita (my first GSD) was so laid back and maternal she mothered both the new pup and the rescue. Both followed her lead in everything they did - to be honest she did most of the work for me.
Not all dogs go through the 'teenage' stage, my Kita was an 'old' dog from when I got her (12 weeks old) and I never really felt like I taught her anything. She took everything in her stride and I never saw any dog willing to take her on - she just had to look at them and they stopped in their tracks. She was a fantastic dog to own.
Would the rescue allow you to 'foster' a dog with a view to becoming permanent if the dog fitted in well?
Sue