Where do I let this dog sleep tonight? (or until we find his owners)?

I posted a question earlier about the chocolate lab I found. I have no knowledge about this breed, but I have had him all day and he is very calm and sweet. I cannot let him sleep in the house because the cats are stressed about it. I cannot let him sleep outside it is cold here in MD. So that leaves the garage (heated). He gets upset when my one old dog leaves the area he is (shep/husky X) and likes to be around him. My dog could take him or leave him. He normally sleeps in the garage on bedding...will it be safe to leave the stray w/ my dog overnight? I feel wary about it. What are the chances of him totally doing a complete turnaround temper-wise?