Hi,
Yes, you can leave your cat at home for 10 hours, but be sure to give her attention and lots of activity when you come home (and before you leave). Most likely she will spend a lot of time sleeping and eating, so she will be ready to play when you get home. You might look for some toys that move when she touches them to help keep her stimulated during the day.
If you need permission to adopt another cat as a friend, you have mine! But there are a lot of "single" cats out there that do just fine on their own.
A good site to look at is www.indoorcat.com . It talks about how to enrich the environment for indoor cats to help prevent stress and behavioral problems.
Hope that helps. Let me know if you have other questions.
Dr. Scarlett
Cat Veterinarian
I am a practicing small animal veterinarian with 13 years experience.