In general I agree with with the smaller footprint, 20mm or 1/72 for buildings. Most of mine are in this scale and I think they look fine. I've even got a few HO scale (1/87) buildings but these usually require a fair bit more work adding height and rescaling roof tiles etc.
The problem arises not so much with figures but when you plonk a 1/50 or 1/48 scale vehicle next to them, especially if it's a big vehicle. If you aren't going to plonk an Abrams outside the building it should be fine.
Helicopters? I have the Skyray CH-46 Sea Knight and it really is perfectly scaled at 1/55. Nice models no mould lines on mine. Do bear in mind that Skyray are box scale. The Blackhawk is 1/60 and the Dauphin is 1/48.
If I was doing your project I'd go with the Skyray 1/60 for a couple of reasons. Firstly, even in hover mode it won't overpower the building and just as importantly, if you want to land or crash land it you'll be able to fit it on the table. Secondly doing a respray of a diecast, even with masking of canopies off, is usually a lot less work than building the average 1/48 scale chopper. It's also a damn site cheaper.