Hi,
The first thing that determines how well they will grow is the climate of where you live. If you are located in the southern states then they should be okay, once it does not get too cold, though I guess you could take them inside and grow them under lights.
Here in Jamaica, the poinciana is a very popular, well loved and beautiful tree. While they are tolerant of areas with high rainfall, they tend to do better in the areas that are drier, hence in the more southern parishes (counties) which are drier, they are very abundant. I have one in training which is about 4 feet tall now, but it has a far way to go based on what I am planning for it.
I have two friends who have more established poincianas which they developed from collected material. I try and get some pics to post later. i think that in order to capture the grace and beauty of these trees then they need to be on a larger scale as bonsai.
I would love to know where you are located and see some pics of your poincianas.
All the best with them.
Nigel