There's a decent discussion about making aluminum wire connections over here: What's the best way of replacing a plug or switch in a house with aluminum wiring?
If there is #12 Al wire on the circuit, the breaker should be 15A. #10 aluminum wire can have a 20A breaker, and you shouldn't normally see anything like #14 aluminum. Or you could just pull new copper wire and permanently disconnect the aluminum. Your home insurance company might not even cover you if they discover that you have aluminum wiring on your branch circuits. Aluminum is still common for the big feeders into your main panel, though, and that's fine because it's terminated appropriately by the electrician.
Any device that aluminum wire is connected to should be approved for connecting to aluminum wire, and any connections between aluminum and copper wire have to be made with special connectors. There are different types. Some are crimps that compress the wires very tightly to "cold weld" them together, some are special wire nuts, and you must use aluminum oxide inhibitor. The oxide inhibitor is included in the special purple wire nuts that are rated to connect aluminum and copper.
One of the issues is that aluminum and copper have different rates of thermal expansion and contraction. Combine that with the tendency for corrosion when aluminum and copper are in constant contact, and the fact that aluminum oxidizes (rusts) very rapidly if oxide inhibitor isn't used. All the inconsistent expansion and contraction and corrosion helps to loosen connections. Add the fact that aluminum oxide is an effective electrical insulator and you have a lot of potential for connections to go bad over time, leading to overheating and fires. Loose electrical connections get hot (and might also produce sparks), and heat starts fires (as do sparks, since they're hot), etc.
If you're going to leave the aluminum wire in place, though, maybe you want to avoid the twist-on connectors (wire nuts) and use the cold-weld crimp-on connectors, then seal the connections up really well with something like lineman's rubber splicing tape.
You don't want this in your walls: