Shubs are still goldfish and get to 9-18 inches long. NOT a good container summer pond.
Any kind of livebearer would do great, how hot does your summer get? Don't get any tiny fish that can't handle the heat, as the water in a container will shoot up with the air temp.
I release my platy in my 3k koi pond. They breed like mad in there and I scoop them all up for winter.
Leaving the hobby failed, sorry. Should of known better.