[Andrés]

Some related question:

But it would be plausible for life, even if they breathe oxygen, to evolve in an atmosphere that is toxic or unbreathable four us, perhaps combined with different G, pressure, temperature, ect...
For example the 70% N2, ~26% O2, ~4% CO2 atmosphere Dogscoff had suggested would not be healthy for humans, but perhaps perfectly fine for an alien species.
That would have similar effects in a sci-fi story without the need of unrealistic chemistry.

What about silicon based lifeforms? Are they possible? Would they have different requirements and tolerances than carbon based life?

In the Star Wars universe there are a few cyanide-breathing aliens, whose atmosphere is as deadly for us as our oxygen-rich atmosphere is deadly for them. Is there any hint of realism here?