The Butia is very spewcial. The usual color of the flowers is red, but there is the rare yellow form. It is amazing in person and I went through a hell of amount of work and money to get a feew of the yellow trees.
The are 15' long are arent? If they are that long I would certianly think they are large enough. Where do you have then? I see you are in 8b which would cause them to freeze back if they are outside. Mine is less then 3 feet long, about 1.5 or 2 inches in diameter at the base.