The purpose of having flexible wedge is to allow for compensation for any angular misalignment between your disc and your seat. If you were to have a solid wedge, and an angular misalignment of 1 degree, for a 20" valve, this is a significant reduction in seating contact, which could potentially result in leakage.
For smaller valves, it is easier to achieve the seat to disc alignment, thus a solid wedge is possible.
My advice, let them know a flexible wedge is the better option and the potential for future maintenance headaches. If they do not agree with you, then give them what they're paying for.