Mike,
The number of diffuser locations depends on a lot of factors. Is your pond round/oval and deep in the middle? If so 1 is probably sufficient. If not, then you'll want to consider more.
I have a Thomas pump, and it is probably a little overkill for a .1 acre pond. Whatever you do with diffusers, keep in mind you're pushing 3.5cfm with that pump so your diffusers need to support more than that, or you're likely to either cause a lot of back pressure on your pump which is bad for them in the long run, or blow out the diffuser membranes. With the vertex for example you'd need >2 diffusers.
