Will these filters work with Windex Dream models?

I use them with a Widex Dream Fusion (behind the ear) model and they are a perfect fit. I can't comment on whether they would fit an In the Ear model.

How often should you replce the wax guards in widex beyond model hearing aids

I replace my Widex wax guards every time I get the "Low Battery" message in my hearing aids. So, when I change out the batteries for new ones I also change out the wax guards, and I clean off any ear wax found on and inside the rubber domes. This process has worked quite well for me over the years.

Do the work for resound hearing aids?

Yes, the Nanocare Wax Guards work just fine. They fit into the small hole easily and do not come out until you dislodge them for replacement. I replace every other time when the battery gets low.