I just started to work with single end reads, which are already trimmed for adapter sequences and quality.
Do I have to trimm the reads now to the same length of e.g. 100nt for mapping them with STAR?
Is there a negative effect, if I don't?
If they are already trimmed for adapters and quality, don't trim more. Trimming will make sequences shorter, and shorter sequences tend to map more to multiple locations.
What is the length range of your reads? I generally keep reads only within a certain range, and discard the shorter reads. For example, for a 100bp dataset, I keep reads from 70-100bp after trimming, and discard the rest.