What CUC inverts on your site should I avoid with an open topped tank? Is it safe to house blue leg hermits with other inverts since they might kill them for their shell?

all hermits, (regardless of what you may have heard), will kill for shells if they need them. Empties that fit that particular species of hermit reduce this pretty well. Open top stay away from zigzags, and probably nerites unless the tank has a lip.