Considering adopting- Please help

We are thinking of adopting an older chihuahua from our humane society. they think he is 12 years old. He came in as a stray.His discription says he's a sweetheart. I wanted to know what older chihuahuas are like, and just anything in general, such as are they easy to houstrain? Are they calmer of friendlier, and how well do they get along with other dogs? We are buying a puppy volgende summer, so do u think the chihuahua would be okay with that?