Flash fiction comics/one shots are definitely a good practice material. In fact, my first two comics I was actually serious about were one shots.
They short--thus doesn't take a lot of time--but could be more impactful than a hundred page series. Additionally, it gives you the chance to familiarise yourself with the basic story graph (intro --> uphill --> conflict --> peak --> downhill --> resolution --> conclusion).
The important thing to remember when making short stories is to focus only on one conflict.
Firstly, pick a topic you want to talk about; be as specific as you can.
I once wrote a short story revolving a handicapped girl, and the focus of the story is how her friend helped her overcome her PTSD over the loss of her father and ability to walk by showing her just how beautiful the ocean can be (she lost both of them due to an incident at the sea).
Then, list down the the major components of the story. A graph will help you in this step.

Or if you wish, you can use a mind map instead.
One shots are definitely not suitable for stories that have a more complex and elaborate plot. Say, for example, a slice of life story about the aforementioned children and how they interact with their neighbours as they go on with their life.
The beauty in one shots, though, are that you can ANY kind of character arc, cut it out of a longer story, and make it an independent story by itself.