Yeah this is a lot of suspension of disbelief, but Star Trek has a bunch of those, so it's not so out of left field. One common solution in SF novels is for al

Yeah this is a lot of suspension of disbelief, but Star Trek has a bunch of those, so it's not so out of left field. One common solution in SF novels is for aliens to be in an atmosphere suit, typically very high tech (thin and floating). In extreme cases, the suit would encase the whole body in gel fluid and also inject it into lungs and other orifices to withstand pressures from different gravities or ship acceleration. In essence, you need a lot of tech to allow diverse aliens to spatially co-exist. Edit: All of this ignores food. Typically, stories solve that with some kind of nano-replicator that can make any combination of chemicals safe for aliens to eat.