As far as Tribes 2 goes, once the item limit is reached, things will simply stop showing up. If you try to place anything, the game will play as though you dropped an item, but you won't be able to see it. At least, that's how it is in Construction.
Part, I'd like to give a shot at prefabbing. Is there anything different about it compared to regular 3D modeling? (Aside from exporting the prefab). Any regulations I should be aware of? Specific program to use? Polycount?