the major point I'm thinking could go wrong is the magma tube/ obsidian dropper thing. If something is conneted to the ground, is there any way will topple over if its way off balance (real world physics wise) eg
Real world physics? What the hell are you talking about? This is Dwarf Fortress. I know not of what physics you speak of.
Your magma dropper thing will stay aloft so long as it is connected to the ground in some fashion, either by supports, floor tiles, stairs, and other constructions. For the most part, it's not an issue unless you're specifically trying to get something to collapse.
What you will need to worry about is dropping the magma in the ocean. If you want to create a ring of obsidian that you can later pump out, you'll have to figure out a way to drop a ring of magma all into the ocean at once. Otherwise, if you simply pour the magma, you'll get a large disk of obsidian supported by a narrow shaft. Not what you're looking for, I don't imagine.
As for magma safe materials: bauxite, all kinds of glass, iron, steel, nickel, and nickel silver are all magma safe. If you place buildings made out of they will not be destroyed by the magma.
You should note, however, that if a building like a bauxite door has a mechanism in it that isn't magma safe, it will be destroyed when the magma enters the tile containing the door.
All constructions, like floors and walls, are magma safe regardless of material.