What is the impact factor of my short communication paper?
Impact factors are assigned to journals not papers. No paper has an impact factor. The impact factor of the journal you published in, and the format makes no difference to that (as long as it is a peer reviewed format).

In my field (molecular biology/genomics/bioinformatics) people absolutely cite short communications.

Once a paper is published, you cannot publish exactly the same work as a full length article. You might publish further analysis of the same data, or the same analysis on different data, or different work that comes to the same conclusions, but you cannot just expand this same work to a new paper. Your new paper must also acknowledge the existance of the previous, and therefore would probably not be seen as as novel as the first one.