So, I decided that my space setting wouldn't have aliens in the extraterrestrial sense (barring one exception), so all the "alien" races of the galaxy are descended from Terran colonists who diverged from baseline humans over time due to either environmental adaptation or genetic modification.

The decision to make them human derived rather than extraterrestrial was because I want my setting to seem scientifically plausible, but not necessarily hard sci fi. I found that true extraterrestrials raise too many questions like how can they co-exist with humans on the same spaceship without one of them dying from what is to them a toxic atmosphere?

I'm wondering how "alien," can these aliens get? I sometimes get a cool idea for an alien race, but don't know if it would fit with the rules of this universe.

One of these race ideas, for example, have green skin and small horns protruding from their craniums.

Where is the line between plausibility and ridiculousness?