Yup, you can put comets in ponds. Each spring my mother would get goldfish or koi and put them in the man0made ponds she dug holes for in the backyard, then come fall we'd move them to the huge natural pond out back. There are a lot of koi/goldfish fish in there, as well as the sunnies and minnows and frogs.
All comets and common goldfish grow from a foot upwards in size. They are really not aquarium fish despite what people at pet stores will try to tell you. They produce a stunting hormone when stressed, which is often what will happen in a cramped or poorly maintained tank. This can reduce their lifespan (over twenty years when properly cared for), and adversely affect their quality of life.
I would be looking at re-homing your fish. Six goldfish in what I assume is an underfiltered and uncycled 2.5 gallon is cruel, and I would not expect any of them to thrive unless you have the resources to commit to a larger tank. I have three Shubunkins in a 46 gallon tank and I know that is overstocking it. But I also do weekly 50% water changes and have two filters that turn over more than 1000 litres of water an hour between them.
Goldfish are not low maintenance, but their hardiness and ability to live in the most squalid of conditions, is sadly exploited by the industry.