I recently took in a cat, who was a joyful little thing. I had to keep her in my garage until i could get her to the vet (the next day) b/c i already have cats in the house. She got really quiet and wouldn't come out of her carrier for me--she was clearly stressed. Once inside the house, she stayed in dd's room for the most part and wouldn't respond to our affection. She tried to get out for about 3 days, which broke our hearts. But after that, she's been great. She seems very happy here where she's always fed, dry, loved and played with. She does, however, appear to be gaining weight. Oh, well. She also tries to get out sometimes.
They will acclimate, and it should only take a few days. I felt really guilty about taking her in (I only did b/c she loved attention and i was afraid of her rubbing up on the wrong person), but then i heard that the average lifespan for an outside cat is only 4 years. Ouch. We did the right thing in this case.