I think we can assume 'concept characters' are 'real'. Even if they do not make it to the actual game, they'll always be in Oddworld's spirit - well, for me at least.
As an interest, the number of species we know about on Oddworld is 31. The number of 'real creatures' by your definition is only 20.
And for all of you, here's that
Powerpoint Presentation I promised y'all.
I do have an apology to offer - Krikits are actually Skrikits. I told you I was getting forgetful. So that's Skrikits, okay, not Krikits.