Maybe someone knowledgeable in this space can tell me, but is a planet's atmospheric pressure reliably proportional to its mass?
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.
I feel you on Star Trek. It always kills me that 90% of species in the quadrant is a bipedal humanoid, although they did address this in "The Chase" (TNG:S6E20). But I digress.
I m a firm believer that non-natural, non-random life forms will dominate the space exploration including robots, augmented humans, bio-robo hybrids and Supra and sub swarms of those.
No. Proof: Venus/Earth.
So that particular aspect is not something to worry about.