Yeah, a little research to make sure Korean flower customs won't make this send the wrong message is advised. Once we settle on a kind of flower that is appropriate to send, I think it might be good to have people send one flower each. Those who want to invest in bouquets can, but floods of single-flower gifts will send a pretty poignant message.
I, unfortunately, don't know Korean customs. I know in Japan, red is even more associated with romance than in America, due to the idea of the "red thread of Fate" that binds together destined lovers.