I am looking into getting a shetland sheepdog...?

I live in Michigan but in the spring i am moving to Scottsdale, AZ and I have to other dogs one is a lab about 50 pounds around 3 years and a flat coat retriever that is about 35 pounds around 7 years and a cat that is around 8 years. I walk my dogs once a day for about 30 minutes and would the dog be good in the heat? The dog would be loved and cared for but would getting this breed be good for the heat and around other dogs? The flat coat retriever is a little territorial.