Generally, keeping things consistent is simplest.

If everyone is wearing the same hire package, asking everyone for the same contribution avoids confusion.

There may occasionally be exceptions. A page boy won’t necessarily cost the same as an adult, and someone requiring different garments or additional alterations may have different costs.

If you’re privately subsidising one member of the wedding party because of their circumstances, there’s no need to announce that to everyone else.

Fair doesn’t always have to mean identical.