With regards to rust in the lines from black iron pipe or galvanized chippings from galvanized pipe, a quality line filter would eliminate any of these particles that could damage the tools. It's a good idea to have a filter regardless of the material type. I have a 5 micron combination filter/regulator at every outlet and have galvanized piping throughout my garage and into my basement. If rust smaller than 5 microns enters my tools, I would think it would be small enough to not affect them and would just blow out the tool.
FWIW, I used cheap fittings and a chinese manual pipe threader with my 3/4" galvanized pipe system and it didn't take but maybe 8 hours to put together 100' or so of my pipe network with 4 drops. There is a very tiny leak at a union that I cannot get sealed, but I shut off the ball valve at the compressor after I'm done using it for the day. I plan on replacing the union to see if that cures the small leak. You don't need an expensive pipe threader if you are doing a small system, unless you are afraid of a little labor. Hell if you are that lazy you can buy the pipe from Lowe's and they will even cut and thread each end for you for free.....or hire someone to install the system for you.
I looked into aluminum but decided to go with galvanized iron pipe after looking at the cost for the fittings for the aluminum system. If you are only wanting a couple drops and don't need a lot of fittings, the systems are pretty similar in cost. The more fittings you need will drive up the cost quick. If money were no object, I would have went with the aluminum just for the ease of installation and ease of expansion. Unless you are branching off an existing tee fitting, expansion is a little more difficult on a iron pipe system.
Regarding the compressor adding to the fire, the compressor would only fuel (oxygen) the fire until it's tank is empty. If you have an 80 or 120gal at 175psi, this could add quite a bit of oxygen. Once empty it cannot create oxygen if the fire has depleted the room of it's supply (assuming the intake from the compressor is not plumbed outside). If the compressor kicks on, it should only take in and blow out smoke. If the line is open during a fire, it is likely that the compressor intake < outlet, so pressure differential of the garage and the outside would only increase from the fire's demands and not the compressor running. To simplify, the compressor alone won't cause additional air to come through the wall joints (increasing the oxygen supply rate). That's my opinion, but I could be wrong.