New dog & older cat

Hi all,

I need some advice, tips, tricks, stories, anything basically. My boyfriend & I are thinking of getting a dog, the one we want to adopt is about 10 months old and a Shepherd mix, so fairly large. His profile says good with cats, but I have an 8 year old cat that has never been around dogs. I did puppy sit an 8 week old puppy for a couple hours once and my cat just kind of kept his distance, but I am wondering if it is because the puppy was still small and not as threatening. I’m scared if I adopt this dog and him & my cat don’t get along or if my cat starts to act out that I would have to return the dog. Any advice? I wish I could have another dog come over to see how it is but I don’t know of any that would be around the same size as the one I am hoping to adopt. Thanks in advance to everyone!