The atmosphere being dense enough to allow for kind of a hovering might be possible, given that life could form in those condition and would evolve some sort of a gas bladder as seen in fish, which I won’t tell you, as I’m not a biologist. But yes, that might work and to be honest, the image of “sky whales” hovering over the landscape while eating airborne plants similar to pollen-like plankton seems pretty cool. This might have been discussed before, I’m not sure.
As for cities built in the air, I’m not quite sure. Free-floating cities seem not too convenient and I don’t see a reason any civ would do it. There would have to be a lot of effort put into constructing a above-ground city without any significant trade-off. Moreover, even a bird-like race would probably not form their cities above ground, as their bird ancestors would have probably been building nests just like our birds. Perhaps they would like cities at elevated places, yes. But I don’t think they would have a reason to construct a city situated in mid-air.
TL;DR - Sky whales cool, hopefully possible. Sky cities, I don’t think so.