I was wondering if it would be unwise to get a very solid foundation in writing by starting with flash fiction, then working my way up to larger works. I suppose I'm being naive in a sense, because I'm sure any decent writer would say that any medium/genre is an art form in itself, but humor me.
Starting with flash fiction alone would involve a number of tradeoffs. On the positive side, the shorter length can seem less intimidating as a goal and can provide more frequent positive feedback from completion. Flash fiction also tends to force the writer to ruthlessly edit the work to meet the strict length limits. Learning to kill one's children (cutting out even a single phrase may seem like murder) and to rework one's writing beyond completion into being finished is useful. Such short fiction may also teach how to imply information rather than showing or telling and to accept unintended interpretations.
On the negative side, such length limits can reduce the fun of writing by restricting the writer's expression and requiring the hard work of aggressive editing.
Flash fiction may also increase the pressure to complete (but not finish) the work in a single sitting. This may be helpful as a discipline to prevent stopping and to force the dedication of significant, undistracted time to writing. However, it may also increase frustration from writer's block and teach a false sense that editing is less important.
I suspect that using a 500 word minimum for a completed work would be more helpful for a beginner in providing a sense of substantial accomplishment without being overwhelming, while allowing more freedom of expression than the 300 to 1,000 word maximum of flash fiction. Taking each first draft and requiring at least a 20% cut in word count in the first series of edits might provide some of the benefit of flash fiction's constrained length.
A common suggestion for beginning writes is to have the means to jot down story ideas and snippets of text. These can be useful in dealing with the dreaded blank page.
Another common suggestion is to dedicate a minimum amount of time to writing and to recognize that research, organization of materials, and editing are part of the writer's work. If a creative block is reached, it is often better to spend the dedicated time editing, organizing and reexamining notes, or even doing research than to stare at a screen. Editing is perhaps the most helpful since it is more likely to stir the writing mood, but sometimes even editing can be unproductive and the other tasks would be more useful and more affirming.
Having multiple stories in progress provides more opportunities to shift between tasks and types of tasks, but there is a danger of having too many uncompleted stories. Having many uncompleted stories can discourage writing by giving the impression of there being too much work to do and of one never completing one's stories.
I think this might be a good idea. Flash fiction allows the writer to just write, unhindered, without the pressure to produce instantly usable material. I think this is a useful exercise and something you may want to hone, as this can definitely free up your writing style when you get to writing the first draft of something substantially longer.