As a follow up, I started with a loop of brass, and then started buying stainless. I built my layout of sectional track, mainly because I was using a program to fit everything in, I'm not a great track planner.
I used all of my brass straights in the layout, and slowly swapped them out as I progressed. A year later after I was all stainless, I noticed that locomotives slowed down a bit on spot on the far side of the layout. There was a 1 foot straight of brass still there! No appreciable corrosion from the clamping to stainless on either side.
The train slowed a bit on that section, even though the track was very clean. Brass oxidizes quickly at my house.
So, I think you can mix for quite a long time with no ill effects.
I too have areas in my railroad outside where I have stainless mixed with the Aristo brass track. I had gotten some before I converted to all battery, and I just can't bring myself to not use it!! I have seen no ill effects at all after several years outside. The grade of stainless that Aristo uses is pretty benign, so I really don't expect to see any thing like galvanic corrosion occuring - and so far it hasn't.