Yea do what you wanna man!! The Ultras (which I love!!) are a very regimented force. They MUST all look the same (within reason!!). But they are OLD as a chapter so you will have LOADS of slight variants in armor markings and the such. Keeping with the fluff You CAN make them as markings or say your a Ultra divergence chapter,not true ultras but desended from them.
good luck with it!!