Or be like me and just don't care about the size difference.
Hey, in my 1st post I super conceded that some people/groups/TO
's will be totally ok with the size difference. That's totally Valid, and I'm not going to tell you/your group/TO
how to model. That's between you and your local community/play group.
I was just commenting that the size difference is enough that I could see people taking issue with running them as cerastus class knights, so I wouldn't just ASSUME everyone is going to be ok with that. Please don't take offence, but it would be comparable to running a Castraferrum Dreadnought with converted weapons as a Contemptor Dreadnought. "Close enough" for some people, to big a difference for others.
Like I mentioned, I would just run the Spear and Board knight as a counts-as knight gallant. Even if they aren't great rule wise, they are fun combat loons all the same.