The cotillion is a formal dance which originated in France and is now popularly used in high society gatherings (ballroom dancing), especially in a young lady's 18th birthday party or debut. The girls here wear floor-length gowns while the boys wear the tuxedo. Ideally, in a grand deb ball, a cotillion should be composed of 18 pairs. But since this is very hard to do the cotillion de honor is performed by 9 pairs with a total of 18 dancers.
Coming up with 18 pairs is an impossibility for it means looking for 36 people to compose the cotillion and who can be present during practices. It is not easy to get a hundred percent attendance in practices so this can be one problem that may affect the total performance of the group.
Budget for the cotillion dancers' gowns should be considered too. If the family decides to rent the gowns and suits, well and good. You can opt to rent the gowns and suits.