Re: Sulu and Chekov, not in Space Seed
I'd go with the opinion that Khan and Chekov meet off screen. It's easy enough to rationalise that with a crew of 430 that Khan meet some of those off screen.
Walter Koenig once joked that Khan was going to the bathroom and Chekov was blocking it. Khan got upset yelling "I will never forget your face!"
A movie aiming low should not be praised for hitting that target.