I am just doing what I do for the gorgonians:
rotifers, oyster eggs, cyclops (too
big for sponges of
course) and now:
Fauna Marin special food for Seafans anf filter feeders.
I like reading about Jens Kallmeyer's tank and his approach. He has been very successful with non photosynthetic stuff (carnations, sea fans, etc.) and while he doesn't endorse any specific brands, it's the FM stuff he uses.
I haven't seen a book on sponges but the "Sea Squirt" issue of Coral Magazine has a lot about Filter Feeders, including a special feature on Kallmeyer's tank.
Yes I was worried about the care for the spider sponge but I got it as a frag from Ace. From what I've been reading the white zoas can live for quite some time even after the host sponge dies so the sponge (again) is the more delicate member of this partnership. I did feel encouraged when Ace posted it's an orange sponge rather than a red. When I'm feeding the corals the white polyps on the spider sponge actually catch and eat very readily.
We'll see. I am hoping with my special foods it will work.