I too have been curious when Mike will start working with Gustavo Freng. Not a spoiler, I don't think, to say that it hasn't happened yet. I lean towards another season or two until they start running parallel.
Another comment on this (how much more is there to say about the Stagg?). This is why I think they are doing such a good job writing the script. There are a lot of things we know will happen or current things that must stop but it's still engaging and a mystery as to how it happens.
* We know Mike works with Gustavo in BB. How do they hook up?
* We know that Hector hates Gustavo for some reason (Hector killed Gustavo by suicide bomb in BB). Why?
* We know that Jimmy McGill changes his name to Saul Goodman at some point. Why? When?
* We know that Jimmy's brother, Chuck, and girlfriend, Kim, are nowhere in BB. What happened to them?
I'm sure there are more, but those are the ones that have me hooked.