I think medium armor is probably good enough for a light tank, especially since we're going to have other defensive systems on it. Heavy armor does come with those penalties to speed and maneuverability, which hurt on vehicles who're going to be trying to take shots from the flanks rather than face-to-face slugging matches.
But trying to be too flexible with this design when we've already got the iterated Light Grav Vehicle filling the role of a flexible chassis is a mistake, I agree.