In a 15gal I would see if you can get smaller ones, maybe frags. They will grow and can grow very large but it will take a long time. You can also just keep fragging them as needed. These are both semi-aggressive corals so give them some space.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian