These are indeed special and You seem to be doing the right thing. If you keep it on the dry side, you will be fine. Both Ken and Hetty are successful with using 100% perlite.
The only reason, it might not do anything now is the plumerias are going into dormancy especially in your area. So keeping your plumeria cuttings media on the dry side is like keeping a cutting for next year's planting. You might get a head start because they could root early with you setting up a grow light. Dry, dry, dry soil with occasional misting on the stem to avoid dehydration would be key to making sure your cuttings will not rot. You can also wrap it with a moist paper towel early in the morning but making sure you remove them that at night so that the stems are kept dry to avoid any fungus.
Post some pics here of your cuttings and set up and we will look at how healthy are the plumeria cuttings you got (tips are shiny if they are healthy and stems would look tough not shriveled). If I were you, I will keep a watchful eye on them and document their progress by taking a photo at least a month (or posting it in this thread). It helps you see (and "us" if you post it here) if there is a drastic change that you should be worried about.
Happy growing
