NGS miRNA size selection for analysis

Hi all,

I have several questions regarding to NGS miRNA analysis:

1. For the NGS small RNA size distribution, the peak shall be at around 22 nt, but what if there is another small peak at 29 nt, is it normal?

2. While doing the analysis on the clear read, I selected these reads that longer than 18 nt for analysis. Is that ok if I select reads at the range of 18-26 nt for the downstream analysis? Why? How this would affect the analysis results (e.g. identification of dysregulated miRNAs, control VS. treament)?

3. How to decide if there is any contamination when looking at my NGS small RNA data?

Highly appreciated for your kind help!

Best,
William