The amount of replies should give you an identication of how amount info is really out there for caring for sponges
If I were to try a sponge again it would be a blue sponge. This is strictly personal observation, but they seem to last longer then tree or ball sponges.
All sponges filter-feed, but definitely not sure what. I don't believe spot feeding phyto helps, though it will not hurt. Personally would broadcast dose phyto in the hopes it ups the nutrient levels in your tank, and basically find a balance between dosing phyto and maintaining water conditions.