Hi Gizpobow,
There is more about the pros and cons of different types of protectors and the tradeoffs involved with each of them in post #89 here. The lanoodles topper is very breathable and it’s unlikely that most people would have any temperature regulation issues sleeping on it but as you mentioned the St Dormier would be more protective while the stretch knit cotton protector would have less effect on the “feel” of the topper.
This would be a matter of personal preference but it would normally be “best practice” yes so that both the topper and the mattress were protected from the body fluids and oils that we release each night and to keep both of them in a clean and hygienic condition because the topper can’t be washed if it gets soiled … only spot cleaned.
Phoenix