You can get circular under gravel filter plates too.
Fancy goldfish aren't easier, they are just more suitable. Comets are short tailed fish that require a lot of length in which to swim as they are very active. They can't do this in a round bowl. The fancy goldfish, fantails, google eyed, orandas etc do not swim much, or fast, so don't need the length to do so.
If you decide on no filter I suggest you use a lot of live plants. Hopefully there is enough lighting to keep them alive.
I would suggest about 10L daily (a bucket) change to start with. May need to be increased as the fish grow. They will grow (do not believe the myth that they only grow to the size of the container) albeit more slowly and will produce even more waste as they get bigger.