Why not use 4 steel grids for the light and 4 grids/32 girders for the heavy

because that would make the light scaffold versions move from using just iron to using iron plus nickel
I was thinking small Still tube as that is essentially what they are. I am tempted to change the heavies to not need grids and just use tubes. but that would sting anyone that has built them already

It sounds good, logic wise but, it seems like it would just be an pain while building, having to add in tubes instead of plates when building a ship. Plates are simpler.

It'd definitely be a pain. I downloaded this to get away from using a scaffold that needs tubes. At eleast go small tubes, you can only carry a handful of large ones.

Griders are the thing. Look at them, those are not tubes, but griders. Plus you can carry a lot of them for all your construction needs. And ye, plates create way too much weight for the ship, while scafolding blocks could be used not as temporary placeholders, but main structure for lightweight bots or something, that's why i vote for griders. (If i'm not mistaken components affect weight of a block, no?)

Girders or small tubes would be fine.
I play a peaceful single player game so there's no reason for me to have ships made out of armor. What I'm doing is filling in the empty spaces with this scaffolding and covering them up with thin armor plates from another mod.
Makes the ship look the same as if it had armor blocks as a hull but (hopefully) much lighter.