I just learned that Jupiter's “surface” gravity (measured at the cloud tops) is only around 2x that of Earth's. This is because even though Jupiter is much mor massive, it's also much larger, meaning the “surface” is much further away from the center. I think that puts me in acceptable fudge territory such that I can believably ignore most of the biological effects of higher gravity. Even more so when you take into account the floating cities will be somewhere below the top of the atmosphere, and you don't feel the gravity of a hollow sphere if you're inside it, the mass of the hollow sphere being the atmosphere above you.

I'm still mulling over whether the cities are sealed or whether they exist at a level in the atmosphere where the pressure is survivable and the mixture of gases breathable. If they're sealed, they would just be orbital colonies with gravity as far as internal design is concerned, so for the sake of uniqueness I'm leaning toward it being breathable.

so-called “terraforming” would be making that breathable layer of the atmosphere. The city would be very much like an oil rig, with extraction equipment hanging from the underside and extending down into the non-breathable but exploitable gas layers.