My SOP is to do switches or outlets one by one, in the sequence I find convenient. I turn off the one circuit it's on.
And then I handle it as if it's energized. (Electrocution requires a path through your body, so you are particularly in danger if your body is also in contact with some other wire or object such as a pipe that is grounded) that would complete the circuit.) So aside from never touching a line conductor, I also make sure my body is touching no other thing, so even if I do bump an energized conductor, nothing happens. I'm not advocating working on energized equipment. I'm saying "belt and suspenders".
I discuss with everyone else in the house about what I'm doing, so they won't be messing with breakers or coming up and touching me and potentially "completing a circuit".
For identifying circuits, a few dollar-store night lights are convenient.
I don't get too formal with my work planning. I leave a marking behind saying "I've been there", piece of masking tape, etc.
As far as aluminum, there's "good" and "bad" aluminum. During postwar reconstruction (or in the US, a housing boom) there was a worldwide copper shortage, so they hastily made household wiring out of the AA-1300 alloys intended for transmission lines. Good conductor, but very bad mechanical characteristics at terminations. Aside from metal interactions, the stuff creeps, so it will "unspring" the force of a firm clamp. The industry fixed this in 1972 with a new alloy, AA-8000 series, specifically made for household wiring, marketed as such, so brazenly marked as such. These are legal today for new work. (NEC 310.14).
If it is not marked, it is a 1300 alloy.
Given the timing, your home might be using the new stuff so it's worth looking. If it is AA-8000 series, I would not rewire, just use parts or pigtails listed for CO-ALR. (Copper-Aluminum Revised; the old CU-AL standard proved insufficient.) There's a lot of scaremongering about aluminum wire, but it is only true for the 1300 series. Nonetheless, you'll take a hit on resale value even with 8000 for 8 AWG and smaller wire. AA-8000 is ok for the big stuff, 4 AWG and up.
With several million homes out there with the bad stuff, there are solutions to fix the termination problems - typically a pigtail, a special splice made to bind the 1300 series to a short length of copper for connection to the fixture proper. I would evaluate those.
The manufacturer of the receptacle should make this detail clear in their documentation. It might even be printed on the case of the receptacle, though perhaps in very abbreviated / "hieroglyphic" form.
That said...
My understanding is that aluminum wiring must be used ONLY with approved mechanical connectors specifically designed for aluminum, and of course must be installed correctly. So I do not see, offhand, why a separate copper connection on the same device would be a problem.
[Note - I am not an electrician. Aluminum wire done incorrectly is a huge fire hazard, which the OP obviously knows since s/he is obviously intending to do this right.]
Best Answer
THHN should be more than fine for a pigtail job; in fact, the wires in Romex aren't intended to be used by themselves i.e. outside of the cable jacket as they have no UL listing for that use -- the cable as a whole is the only thing that carries the listing.
(THHN/THWN is what's used inside metal-clad cables and when wiring conduits -- it's the "standard" wire as far as individual wires go for building-wiring use.)