My brain keeps saying something like conductors of ther same type and insulation but I think that relates to Class 2 running with power.
You might be thinking of parallel conductors, where they have to be as identical as possible within a given part of the circuit (phase, polarity, neutral, egc). The conductor metal, the insulation, the size, the length, and the conditions of installation, all have to be identical to prevent any one conductor in a parallel group from becoming a preferred path over its partners, and not dividing the current equally.
It's OK to mix different types of wires for circuits in general, as long as everything is sized appropriately, and has the necessary ratings that apply. In most applications I'm familiar with, XHHW-2 can be substituted for THWN-2 without issue, but not necessarily vice-versa, since XHHW-2 has a few subtle advantages over its THWN-2 counterpart. As long as you aren't depending on one of those advantages, the two types are interchangeable.
