Scaling models in Unity - Bad practice?

Up until now, my artist and I have been creating our game levels in 3DS Max and exporting each level as a giant FBX into Unity. Neither of us have any game-design training, so we thought this was the correct approach but recently I've had a couple of people tell me that if we instead export each component of the level into individual FBX exports and then piece together the level inside Unity, we gain optimization benefits. (E.g. using one tree prefab and duplicating it to make a forest will count as only one tree in memory, whereas creating a forest in 3DS Max will count as thirty trees in Unity memory.)

Before we move to doing this, I'm wondering one thing about scaling models. I've read in the past that it's bad to scale a model in Unity as it duplicates the model within memory. Is that still true? For the forest example I would probably re-scale some of the trees to be larger or smaller. But if scaling is bad practice, would it be better for my artist to scale the tree in 3DS Max and then export a few different scaled versions of the tree (e.g., tree_big, tree_small, etc.) so I can avoid any scaling in Unity. This seems kinda ridiculous to me, but is it the more optimal approach?