There are no absolute rules in acoustic design. You can ask five different acoustic engineers the same question and get five different answers. On the surface this may appear confusing, but if you dig deeper, you'll find that each engineer has a design philosophy. Each element of the room and the reproduction equipment are designed to synergistically interact in in accordance with that design philosophy. You can get into trouble if you mix and match individual design elements from different engineers. If you decide to get professional acoustic engineering help, you will need to determine the engineers philosophy and make sure it's compatible with your goals.
The issue of surface rigidity is complex. In general, for home music reproduction rigid, but not overy rigid construction is best. Some wall flexibility can lessen standing wave problems. This will give you flexibility if in the future you switch to a different speaker with different low frequency tuning. In pro studios hyper-rigid wall construction is preferred. In a studio every element of the system is optimized to work together. The engineer knows exactly what speaker will be used, where it will be positioned and where the listner will sit. It's optimized, but it's somewhat inflexible.
The issue of surface rigidity is complex. In general, for home music reproduction rigid, but not overy rigid construction is best. Some wall flexibility can lessen standing wave problems. This will give you flexibility if in the future you switch to a different speaker with different low frequency tuning. In pro studios hyper-rigid wall construction is preferred. In a studio every element of the system is optimized to work together. The engineer knows exactly what speaker will be used, where it will be positioned and where the listner will sit. It's optimized, but it's somewhat inflexible.