In the story I'm working on, a rift in space time opened up between our world and a layer of oxygen in a gas giant at some point in the past. Eons earlier, a species of fish symbiotically linked with algae (which lived in the fishes' back, providing nutrients in exchange for protection) mutated in a way that allowed the fish to fill its swim bladder with methane, enough so that the fish could float in the air like a balloon if it voided itself of everything in its digestive tract. However, because this mutation enabled the fish to escape predators and avoid competition, it became more and more prominent in the species, and, due to the square-cube law, the creatures themselves became larger and larger in order to have a higher lift gas/weight ratio, until entirely non-aquatic, self sustaining, living gasbags ruled the skies. A few of these creatures ended up floating into the rift, where they propagated in the oxygen laden atmosphere. They were followed by birds, insects, and whatever else could sustain itself off the gasbags, including a few humans. These early settlers, first hunting of the backs of the creatures, were eventually able to create leather and bone, methane lifted crafts of their own, congregating in small aerial villages around supplies of communal lift gas.
So, could something like this realistically happen? Besides the rift between worlds, I'd like the story to be bound by hard science as much as possible. The main problems I can see are...
The Planet: The planet is almost entirely gaseous, with different layers of atmosphere and a small, rocky core. Could this planet have any sort of magnetosphere? Having wind and a precipitation cycle would be good story wise, but the only way I can see that happening is through the lower layers of the atmosphere being heated and creating convection currents, but what could cause that? Could all of this happen while maintaining a gravity less than/equal to Earth's?
The Biosphere: Would this kind of environment be livable? Even if it were, what would happen to all the biomass? Would it slowly just leech of into the void as creatures died and shed skin cells? What about all those different minerals that are necessary for life, like iron? Meteorites might work, but I believe they would be too rare to support anything.
The Humans: With the resources and technology humans would have available, could they create their own airworthy craft? How long would their supplies of lift gas last? Would trade between villages by means of smaller vehicles be possible, or would everyone be bound to the villages? Perhaps there's a layer of lighter gas above the oxygen that they could harvest, but could they stay completely separate? Would the ecosystem even be able to sustain humans in the first place?
Please feel free to point out any problems you see with scenario.