The winged twilight crashes the CS for me whenever I try to preview it or place it in a cell. Haven't tested it in game because I guess it will be several levels before I expect to see one.
I do not have the same problem. I get some errors when previewing, but not a crash. I can also add them to cells.
I would suggest completely deleting existing meshes and textures and then starting over again with a new download. Make sure you are using the most current version available, although I do not know one way or the other whether there have been more than one of these. I don't think Eloth has updated these with Havoc yet.
To test them you could make a copy of one and give them lower stats then place them where you can run into them. They are very cool. I've run into them in Ilunibi.
Fortunately, with creatures there is always the option of making a leveled list and placing that instead.
Just make a new Leveled list with 0% Chance none and Winged Twilight available at 1st level. You can then place these instead of creatures. Better test these in-game before you do much else. If the CS crashes for you it is probably a good idea to make she the game doesn't crash too.