Is it safe to roller-skate on sidewalks in USA?

(I'm unsure of the legality of roller-skating as transport in these cities.) It's not necessarily frowned upon, just extremely unusual (roller-skating as a means of transport is uncommon in the USA). Note that sidewalks in all of these cities may be quite irregular, i.e. bumpy -- the norm is concrete slabs, which may have fissures between them. Also common are asphalt sidewalks, which are intended to be smoother, but inevitably get broken up and occasionally potholed by tree roots and weathering.

I don't roller-skate, but I imagine that those properties of sidewalks would probably be significant obstacles to roller-skating. That's not to mention that the sidewalks in some of these cities (e.g. downtown Manhattan) can get extremely crowded, to the point that roller-skating freely is practically impossible.

Note, however, that there do exist paths that you could use for roller-skating: usually these are recreational trails for general jogging/cycling/etc. However, as a means of transport, I think you won't find your roller-skates very useful in the USA.

Finally, to answer your question of is it safe: well, it probably won't kill you to go roller-skating in these cities. It'll probably just be quite inconvenient.

Inline Skates would be more practical to use in the USA because they would provide a smoother ride than Quad style Skates.

The larger the wheels are on Inline Skates the smoother the ride with them will be, however they will require more effort to get up to speed, and the top speed of the larger wheels will be quite a bit faster than smaller wheels, so if you are not experienced with higher speeds on skates, you should probably not go with anything above 90mm.

Something else to consider is hills, if the cities that you are visiting have enough hills, you might want to consider how much experience you have with hills while riding on skates, because gravity will assist you greatly on hills and can very easily land you in the hospital if you fall while traveling at the higher speeds that the gravity assist will provide.

If you do this, I'd advise you to be extraordinarily careful at intersections. Motorists making turns are, at best, looking for pedestrians moving at normal walking speeds in crosswalks. A right turning driver isn't necessarily going to see you speeding up to the corner from behind on your skates. This kind of accident is depressingly common when people ride bikes on sidewalks; the same basic risk applies to skates.