Just for reference, here's what I use Poret for. I spoke directly with Stephan about where to start, and it's what he recommended.
For Mattenfilters:
10/15/20L/33 gallon tanks: 2 inch, 30 ppi
40 breeders: 3 inch, 30 ppi
For my canister pre-filters, I use 20 ppi cube material. The larger ppi stuff is less likely to compress if you don't clean them very often (like me). I used to have the gray, ATI Hydrosponges on the canisters....I still use the plastic sponge holder parts and cut the Poret to fit on them.
I've also used some of the blue, really coarse 10 ppi foam in the back of my HOBs as a replacement for their regular filter inserts and in my canister filters. I've tried some of the higher ppi stuff (smaller pore size) in there as well and although it will work in a canister filter, it's more likely to clog and I end up with a lot of bypass around the foam if I don't clean it often (which I don't....oh well)
Also, remember, since this stuff is expensive, if you have good sized pieces left over, they can be glued together with silicone to make a larger piece and they'll still work just fine (according to Stephan).