I'll ask one of our Infinite Scroll experts to have a look, and get back to you as soon as possible.
I'm afraid I have some bad news: you won't be able to use both plugins at the same time for now. We're working on some improvements to the Infinite Scroll module at the moment, and once we releases these changes you will be able to tell Infinite Scroll what query arguments to use and where to load new content.
Until then, I would recommend that you customize the Ajax plugin to load content much like the Infinite Scroll does.