Would it have been possible for Jonathan Harris to tone it down, so to speak? We're all familiar with his "over the top" style of Lost in Space acting...you know, THE PAIN, THE PAIN and all that. I'm not sure that would have been appropriate for Trek.
The early episodes he was much less absurd, much more villain-like, so he could have done it. I see him as an Andorian. Or a Changeling.