I don\'t think there is anything rules wise about it. Anything bigger than a 25mm base I think is considered a large target or whatever the terminology. SO you should be fine using whatever base you feel comfortable with. Make sure it balances the figure well so that it doesn\'t tip over.
I don\'t think there\'s any sort of set standard for FW greater demons in either setting. Maybe I\'m wrong but I think they\'re sort of \"Unofficial\" and the base is whatever you want. I use a FW GUO for fantasy and I give the opponent the option for me to use it on the base I have for it, or I\'ve also got a marked off area for the 50 x 50 \"Official\" Base.