Was My Greyhound Mix Beat?

Ok so a couple months ago i had to male dogs, Mutley and Buster. Buster died and it left Mutley Lonely. i always wanted to find him a new friend to play with but i never knew where to look.
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As you know Mutley is a MUT, he is lab, boxer, and pitbull mix, he is a good size. more muscular looking dog. So yesterday i came upon this ad in the paper that said free to good home black lab mix. i called and met up to see the dog. the dog doesn’t look a bit of lab to me…. but i decided to take her anyway so my mutley would have a friend.
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i got her home yesterday and noticed she was very scared, (of course it’s a new environment for her.) but she seemed to have clicked and got the hang of things fast. Mutley and the new dog (Shadow approx 1yr old) love to rough house. but i’ve noticed she is on the skinny side (i think it’s the greyhound in her) and when she is walking around the house while i am sweeping she suddenly stops and rolls over on her back. when you make a fast move she just suddenly ducks down. she isnt trained much at all, all she knows is to sit. she doesn’t even have the knowledge of how to walk on a leash. i have a big fenced in yard, and i was wondering if you thought she may have been beat too, also what are some good training tips for adult dogs that were never taught as a puppy? 10 points for best informative answer. thank you:)