Considering a couple different things regarding my AI story arc, and I'm curious how defaulting works with regards to Matrix actions. For example, your basic hacking roll is Hacking+Exploit. But what happens if you don't have the Hacking skill? Is it the basic Exploit-1 roll, no roll at all, or something else?

Well since you can actually default on the Hacking skill (why exactly?), it should follow the rules for defaulting by using the attribute -1 as a pool. So Exploit -1.