Mike, I don't think there's a rule about FB/RC - but it should be something that you won't be ashamed of when you receive the other prints...
Some mount, some don't. I don't, and prefer unmounted prints since the stiff mounts are far too vulnerable in the post. An unmounted print between (at least) two layers of cardboard has the best chance of surviving transatlantic post.
For print size see FB/RC - same advice applies. Also consider shipping - a 4"x6" FB print mounted on 2mm aluminium could get expensive
Yuo send some, receive some, and get to keep everything you get. Absolutely wonderful!