They can, actually, as they're used as various checkpoint stations throughout the games. [Best example being Saffron City]
But if you're talking about the default houses, yes, they too can be expanded. That's what the windows are for.
No, quite frankly it isn't, I used the basic numbers of Saffron City + about 5 or 10 on each side, and didn't waste the space with trees on the borders.
Not entirely sure which buildings you're referring to here, but I'll assume the standard buildings with the Grey roof. I personally don't think they're too thin, but it's a matter of opinion here.
Why thank you I might consider moving the roads around a bit.
Thanks for the input Much appreciated.