Oh, by all means. Hell once I switched to a new rules set mid campaign including a few very serious houserules. I should note that this did lose me a player though. I should note that I don't regret my decision, though I do regret how I handled it. Personally I left things without fixing them for too long, and the player that left was actually a huge problem, and part of the reason I didn't change things right away. This does not appear to be the case with you, that's very good, and fiddling with rules is part of the GM's job.
My point however is to not go overboard. In my opinion it would be fine to nerf focuses (people will stop using them very much, but that's fine), or do what I suggested and force stress to last longer in some way. However doing both would be over board. Doing both and nerfing specialization in the same way, would be far too much. One thing to remember is that wizard is a massive investment, and if you take away everything that makes -7 refresh (and a few other things) worthwhile then you may get resentment.
If you do want to implement many nerfs then I would recommend implementing them one at a time, let a story play out and see what happens, see how one nerf plays, then if you still feel more is necessary, implement another, until satisfied. This way you can also gauge if your houserules are affecting the fun of the players most directly affected.
Anywho sorry if I came across strong, this is just something that I've seen, I've done, I've been on the receiving end, and it can really affect some people. It's really good that it's only six sessions in.