Chickens can be surprisingly hardy. As long as they are out of drafts and have sufficient bedding, your Seramas should be fine even if it drops to around 0 F. I don't have Seramas, but my bantams do fine even when it gets very cold. The only problem you may face is frostbite. Serama roosters have rather large combs, so if it gets below freezing, they may get some frostbite. To reduce the chances of this, make sure that the coop has adequate ventilation and put some petroleum jelly on the comb and wattles.
