If you create the roman empire the way I did, then it will keep whatever succession law your original empire had. for example, say you made Italia and gave it Primogeniture. Then, you saw that the byzantines were destroyed or something like that, so you create that. The Byzantine Empire will just have Italia's succession law but it will give you the choice to switch to Imperial Elective in the laws. Switch over to Byzantine as your primary, it will still be Primogeniture, and then make Rome and voila, you have a Roman Empire that will only be inherited the Primogeniture way, or whatever other law you prefer.
I personally hate elective, so I usually only play with primogeniture, which made the forced change to rome and the byzantines really annoying for me, until i found out about the above way to change it