So awesome to hear you found a Rouen she'll be great for him may even tame him down some since she is older. [when it come to mating that is] ;)
Looking forward to seeing the happy couple.
Any breed except Bantam would be fine. A bantam duck would be way too small for a Rouen drake. The new female should be at least laying age which is around 5 months.
Very sorry for your loss.