Congrats on being able to adopt Misty!

I hope that you can try to convince your mom to allow Misty to be indoors only. It will be much safer for her. Maybe you can try confining her to one room with access to a scratching post and several toys? Then when you're sure she's not destructive, she could get run of the whole house.

Even though Squeekers doesn't like other cats, they could still potentially live peacefully in the same household together. If you had Misty confined to one room for awhile, that would help Squeekers get used to her scent and the introduction would go a bit more smoothly.