Can I twist copper on legacy aluminium wire using regular wire nuts or do I need to procure aluminium wire? The wires coming into my switch box are too short and I would need to extend them.
Jumbo Pigtails are the answer here
Ordinarily, I'd recommend using AlumiConn lugs torqued to spec for a copper to aluminum pigtail job like yours; however, since your wires are already too short as-is, installing any sort of lug is going to be a pain in the arse.
So, your best bet is to install another (a single gang old work box should suffice, but run the box fill numbers to be sure) box "upcable" from where the culprit cable comes into the box, pull the old cable out of the old box, install it into the new box, strip back the jacket a bit, and terminate the ends of the old cables into AlumiConns. You can then use some regular copper NM as a "jumbo pigtail" from the box with the AlumiConns in it to the existing switch box. Put a blank cover on the new box, and you're set.