I can't answer your question with absolute certainty, but I would be shocked if Steve wanted the AI to ignore a 100kt diplomatic ship.
Also, you probably don't want to put shields on your diplo ship, since that would make it a military vessel, and therefore it would be subject to breakdowns and require maintenance.
There's really not much reason to make a diplo ship larger than 10kt.
Those are interesting points but I think it would be a pity not to use the available opportunity a diplomatic ship provides, I am a bit of a noob so cannot speak with great authority but that seems like half the fun to me.
Like Russian "trawlers" of the cold war you can hide ELINT modules in a diplomacy module ship (which makes it military for maintenance anyway) and get intel on the target race for a minimal diplomatic hit, if not diplomatic inprovement, because you get the diplomacy and commercial engine deductions on size penalties for diplomacy and can field an ELINT ship without causing trouble and hopefully add in a diplomatic commander to improve relations rather than sour them.
Coupled to which, if the alien is a xenophobe you would want your service personnel to have a reasonable chance to survive if they were attacked, so speed enough to make an escape is worth adding which with commercial engines requires a large one and including passive defence that takes it well over 10kt already.
Whichever way the calc is made a 20kt diplomat with commercial engines will be treated as the minimum rated ship size for sure. So may as well use it all to best effect IMHO.
I mean, yes, you could do that.
But you can already field an ELINT ship without causing any trouble.
With not much research into thermal shielding, it is trivial to spy on alien populations without being seen.
My own diplo ship is 70% engine. It's just a diplo module, three 50-ton scanners, engines, and fuel. Just over 7kt total. I don't see any reason to make the ship bigger just because I can do so without taking an additional diplomatic penalty. I would rather have a cost-effective ship that is well suited for its mission, which is to sit in one place and talk to aliens without making them mad.
It's only going to survive an attack if it can outrun it, but that's true even if I add token armor and/or point defense.
And it's not going to get much faster by using more/bigger engines.
It's only going to get much faster by using more powerful (and expensive) engines, which would make it a military ship, which makes production more costly (because naval shipyard instead of commercial) and means I have to pay for ongoing maintenance.