Question one: I'm just wondering if they will still mature to blooming size this late in the season.

If you are putting them in a greenhouse, perhaps yes, they should bloom around Thanksgiving, depending on how warm the greenhouse is.

Question two: Will my plumerias always set flowers this late in the season?

You don't say what variety you have or how large the plant it. Generally, as the plants get larger, beefier and more established, the likelihood of earlier-developing inflos occurs. Especially if they are being grown in a warm greenhouse.

Also, your plant will eventually develop more growing tips. These will develop at various speeds and stages. The more growing tips you have, the more likely your plant is to bloom at different times, including early/late.

It might depend on your climate, even changing seasonally. Here in early October, every tree I have has blooms and some were late to get started. As I suggested, it depends on your climate. Most of mine start inflows in June and continue to produce even as the weather cools. Then too, as mentioned, some varieties behave differently, so it's kinda hard to just speak in generalities. El Bee

PS it's also true that some plants will always lag behind others, year to year.