I've found that you can equip Serana with about any armor that you want (including custom armor in the Steam workshop)...on the PC it does require some console commands to be used. Usually for a follower to wear items it has to be a higher rating, but from the console you can removeallitems 0 to drop the vampire armor (in the case of Serana). Once that is done you could openactorcontainer 1 and put into the inventory whatever armor you want the follower to wear. I found a good example of Serana with custom armor (along with some other followers):
http://www.virtualsecrets.com/misc_imgs/skyrim-followers-armor-large.jpg