Certified GKH Gardening Expert
Plastic is better than clay for plumerias. Their roots tend to grow into the sides of clay pots, I've read. Nursery pots are ideal because they have a lot of holes in the bottom. If the plastic pot you have has plenty of holes for drainage it will be fine. You can also add holes if there is only one in the center.
I like to read the growing instructions on all the plumeria society sites as well as some of the plumeria retail sites. In addition, there is a Facebook group called Plumeria Growers that has good information.
The Plumeria Society prefers the multiple sized, rigid, black, common plastic nursery containers with at least four water drainage holes towards the bottom to allow for excellent soil drainage. Salt buildup is minimized with plastic and the multiple drainage holes are much less likely to develop drainage problem than pots with a single hole in the bottom center. With clay, single hole pots, the tap root can fill the hole and limit water drainage. This can be a problem for long periods of time as it impairs water drainage and can promote root rot. Feel free to use other materials such as ceramic or plastic with one hole as long as you devote some time throughout the year to verify that there is no salt build up and that the water drainage holes are working correctly and are not getting clogged up.