For instance, I pick Defiance from the Might set and then pick Personal Force Field and Invulnerability later on. Will their effects stack? Or does one cancel the other, so that you are better off taking only one?

You only get one slot for slotted passives, so while you could take both Defiance and Personal Force Field, you could only have one running at a time.

The closest you can get to running multiple ones is having different passives in each build and switching between them as you need (e.g. start in a build with Personal Forcefield, switch to Defiance when the force field runs out, then switch to Regen if your health gets low). Some powers will give you a single stack of Defiance too, which will stack with other passives.

Ok thanks that helps. I think I will stick with Defiance for now at least, seeing as it is tailored for the Might set and being thats what I'm running.