I've decided to allow medium weapons on light vehicles and heavy weapons on medium vehicles. Super-heavy weapons will still require super-heavy vehicles.
In the test campaign, I have reached the point (9 years in) where the various houses are designing their first ground forces. As I started to do research for the planned forces, I quickly realised there are plenty of examples where heavier weapons are mounted on lighter vehicles, creating greater firepower at lower cost, but with greater vulnerability.
As an example of the forces being created, below is the initial ground formation for House Reichmann. This is the smallest of the various house formations and is intended to function as part of battalion or regimental-size formation. House Reichmann is aiming for a mobile, capable offensive force and will have dedicated Panzer formations as well.
In contrast, here is a much simpler infantry-focused formation from House Aurelius, intended to be one of ten cohorts in a full Legion.
House Fuchida is using a larger base formation, with the Japanese WW2 infantry battalion as a template. The individual Fuchida Infantryman is cheaper but less well protected than the Reichmann Panzergrenadier or the Aurelius Legionary.
House Ragnar is infantry-led but uses the low-cost Thralls to soak up damage while the Berserkers do most of the offensive damage.
Finally, the House Varenne Regiment of Foot is primarily a defensive formation with limited heavy weapons. It is intended to hold the line while heavy armoured cavalry forces (yet to be designed) launch the decisive attack.