Since tardigrades are almost extremophilic, and so are these, should we use them as shielding material? A thick enough layer outside the spacecraft will shield it and we may use breeding techniques so that the shield is automatically renewed.
This would not be very feasible. There are several reasons. First, living creatures are unpredictable and uncontrollable. How could you be sure that all your tardigrades would maintain an even layer across the ship? What if they chose to all group up at one location?
Second, because tardigrades are not as dense as, say, a sheet of iron, you would need to have an extremely thick layer of them to provide adequate shielding.
Third, in order to survive extremely harsh climates, tardigrades undergo a form of stasis where they suspend all metabolic processes. So they would not be reproducing at all. And even though they have extremely high survivability, studies where tardigrades were exposed to outer space for 10 days yielded a ~30% mortality rate. So you would still have lots of them die.
Finally, even if they did maintain their metabolic processes, you would still need to provide enough food for them to survive and reproduce. So you'd need to have even more material sent up. Moreover, while tardigrades can survive extremely high doses of radiation, tardigrade eggs exposed to radiation have extremely low hatch rates.
Ultimately, this plan would be extremely complicated, not very useful, and not have any real advantages.