Welcome to the club!
As for the smaller turbo being the optimum size, probably not. But it will spool up faster than a bigger one. You might not make as much top end power but it will still be a hoot to drive. I'd say go for it and get a bigger one if this one doesn't make you happy.
I think I follow what you are thinking with regards to the aluminum radiator. You want to blow the air through where the water is supposed to go? If so, I would say the pressure losses and restrictions would outweigh any cooling benefits. Intercoolers generally have huge internal passages for a reason. I could be wrong though, it might work just fine.
Best of luck to you, and welcome again.