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 o friendlier, and how well do they get along with other dogs? We are buying a perrito, cachorro siguiente summer, so do tu think the chihuahua would be okay with that?