How could I submit it?
The length guidelines are generally for maximum length. You can submit a "long paper" that is shorter than the maximum length allowed for a long paper.
I've published papers 25% under limit without any comments from reviewers on the length. If there is enough original research content for consideration as a full paper, you should be OK.
However, you should consider whether it actually is substantial enough for a full-length paper if it's so short (your advisor can help you make this determination).
Whatever you do, don't pad the paper with unnecessary text (fluff) and/or giant images to fill pages so it looks longer. This lowers the quality of your paper. (As a reviewer, I have seen authors do this and it looks terrible.) If you decide you need to add to your paper to make it substantial enough to be a long paper, then you have to add substantial content.
Could I submit it as both?
Not unless the conference explicitly says this is allowed (usually it isn't). Submitting the same (or a similar) paper as both a long and short paper will get both papers rejected as duplicate submissions.
If I submit is as long could it be accepted as short if it were rejected as long?
This depends on the conference. It will either say something like, "Rejected long papers will not be considered as short papers" or "Rejected long papers may be considered as short papers."