The only change in behavior that I have personally noticed with the addition of a new house cat is a change in play habits. This happened twice and both times it was the same "game" (fetch). After the addition of Sterling, Jasper stopped playing the fetch game with us. (It also happened earlier with our cat Brighteyes). But I never recall any change that I could not live with.
Now this is not to say that Marius won't change, but you have a bond with him and he has one with you and that will never change, and you could not predict it even if you wanted to. I am glad Sergei will be part of your family.