Hi,
I imagined one starfield with random repartition of multiple sprites, but several starfields, each with one different sprite, would do the job, and would allow more possibilities (one starfield for blood cells, plus one starfield for viruses, or clouds + airplanes + rain, etc... well, perhaps a full 3D shootmup could be done with your lib only!).
Anyway, thanks again for your lib, that's a great job!