More plants is always good. As far as "fasting" fry, it generally isn't recommended. BUT, with that said, I've been fasting my tank regularly, and I've had several corydoras fry grow up in my tank with NO intervention from me. The live plants actually are home to a variety of microorganisms that normally would feed the fry in the wild, things like "infusoria" and the like. I have a fairly good number of real plants and the fry do just fine without any special treatment from me.
A 20% PWC won't do much to lower high levels of any contaminant in the tank. Imagine you have 100ppm nitrate, a 20% water change (with 0ppm nitrate tap water) would only lower the levels to 80ppm. Not noticeable on the API test kit. On the other hand, doing a 50% PWC would lower the levels to 50ppm, and a 75% PWC would drop it to 25ppm. Personally, I am a fan of LARGE water changes. The 20% water change schedule that is so often discussed is best for lowly stocked tanks. More heavily stocked tanks really benefit from a much more aggressive water change schedule. I do 25% weekly, a 50% monthly, and a 75% every 6 months. In the summer I do even MORE water changes than that, and do a 25% water change twice a week, sometimes 3 times a week. I use this water for my flowers in the summer, and I use it to emulate a "rainy" season for the fish. It works for me, and I think the fish actually appreciate the change in seasons like this.