It's not ideal as the flash strap pushes the curb chain up onto the jawbone.
If you do use the flash, try to use a lip strap on the pelham as this will help keep the chain down, also do the chain up so that it goes through the rings rather than behind - this will also keep the chain down and is actuallly the correct way to do up the chain.
See pic
Yes you can use a flash noseband with a curb, but it makes the pelham action stronger Horse opening mouth could mean that horse is evading the pelham so dont think flash will help just cos you more probs further down the line.
What mouthpiece is your pelham? If it is single jointed it may well encourage the horse to open it's mouth. I use a myler pelham and the Appy who needs a drop noseband for dressage is very happy with it.