5ppm nitrate is perfectly fine. In typical aquarium people simply keep it at below 40ppm. It is said that for sensitive fish 20ppm is better but no one's fish or shrimp had died to 40ppm nitrate as I know. There is no question though lower is always better, since you will hardly find 5ppm nitrate concentration in a natural environment.
If you want to keep nitrate close to 0ppm, 10% water change once a week might not be enough. I do 30% weekly partial water change to each of my tanks, and all of them have nitrate of 10~20ppm. Although the bio-load in my tanks are a lot heavier.
I got my shrimps from aquabid. Red Cherry shrimps were $14 shipped for 10. Yellow shrimps were $30 shipped for 13.
Yeah, any of these dwarf shrimps will breed fast if the condition is ideal. It took only 3~4 months for 13 of my Yellow shrimps turned to 100~150. I basically relocated 50 of them to another aquarium, and I didn't even notice any less shrimps.
