Pig-tailing 1970 aluminum wiring, choices of type of copper pig-tail
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.)