Maybe someone could educate me. In scuba diving we constantly use compressed air and nitrogen mixes. If it's "compatible" enough to breath at pressure I can't begin to imagine how using compressed air to top of your tires could possibly be incompatible with nitrogen filled tires.
I'm not very educated about it in tire use, but I have extensive training in compressed gases. From what I know, about the only advantage I could see in using nitrogen in a tire would be that nearly all moisture content is removed... whereas with compressed air, there generally is not much done in the way of filtering/processing of the O2. That has a tendency to introduce quite a bit of moisture into a tire. If you've ever watched while a tire is removed from a rim, it's not uncommon to find accumulated water inside of a tire... or for that matter... anyone using an air compressor I'm sure you have seen how much moisture accumulates rather quickly under pressure.
The molecule size thing... I don't know if I buy it as being a significant advantage. I know O2 has one more proton and is around 292 picometers in size vs N2 at 300 picometers. Hardly significant.
I guess I could see less moisture sloshing around in your tire and on your rims as a benefit, but I've personally never put on new tires due to internal oxidation (only worn treads). I would "think" that external oxidation rates would be exponentially higher than inside anyway.
Is it worth the expense at $5 a tire for a top off? Maybe I'm missing something.