Great choice with the Mardi Gras, it is a blooming machine. You mean, will it block the KO rose in back? How big is the rose? My KO rose has never gotten much bigger than two and a half feet tall or so, due to all the winter dieback. My Mardi Gras Helenium gets around three feet tall, maybe a smidge more. If your KO is under four feet I would say it would probably be blocked by the Helenium (which has never fopped for me btw).
Hope this helps ;-)
CMK

The KO roses are young, still only 2 feet tall. Although I don't usually see dieback on other roses I have in backyard. That being said, perhaps I should move the KO roses to the front and the Helenium to the back? It looks like it only flowers on the top anyway right?

Do you pinch for height control and still get 3ft tall?

I don't pinch my Mardi Gras, it naturally gets three feet tall or a bit more. Yes, it is only the top most part that blooms.

Hummm...it is really hard to say. Most roses over here have a ton of dieback, so many never reach their full potential. I looked online and double knockouts are said to get 3-4 feet around. That is cutting it close since that is about the same height as the Helenium. Since you don't get a lot of winter dieback you might expect the rose to get four feet, but again I am not one hundred percent sure.

Sorry I couldn't give you any pat answer ;-(
CMK