I saw this video of a lecture by Feynman where he said that electrons behave like particles when there is a photon source to detect which slit they pass through. Does this imply that electrons mostly behave like particles, since the real world is full of such possible interactions that would destroy interference?
The short answer (I hope someone gives a longer, more precise one) is that not all photons are equal when you consider this question. You need a photon with a short enough wavelength (high enough energy) to measure the position of the electron precisely enough to make the uncertainty in the position less than the spacing between the slits. Most photons aren't that high energy, so electrons aren't forced to behave like particles most of the time.
Please note that I'm being very loose in my language (at about the level of the question, I think). A much more precise answer is possible, if you're interested, but it requires a lot of clarifying of concepts covered in the original question.