Ava looks like a Sheltie so I googled "are sheltie's good with cats" and many links came up. This thread from dogster had a lot of good input.

http://www.dogster.com/forums/Shetla.../thread/460905

I think if she ignores Mizzie or lets Mizzie be boss, everything will work out eventually. I never thought my Moon would tolerate living with a dog, or even another cat. Surprisingly, He loves having fursibs!!