Did you aclimatize them to your tank using drip method? Inverts are more sensitive to salinity and Ph compared to fishes. You cannot just dump them into your tank.
Feather worm you can just squeeze between rocks. For cocoworms since they have hard tubes, you can just place them anywhere. But take note that cocoworms have short lifespans. Max 2 yrs like that.
What kind of microscope are you using? I've use optical microscopes in semicon factory and they can go up to 1000x without any oil. But the lens is very close to the sample already.