Just need some advice,
Would you consider using a mix of Primaris and Firstborn Marines in the same force?
I'm thinking of a Scars successor (to avoid painting white) as all-mobile as possible.
In this instance would you have the Primaris as the 'Assault' element or the 'Line Holder' role?
There are only a couple of reasons not to mix them
1) you don't like how it looks.
2) your worried about firstborn being squatted.
Point 1 is extremely subjective, point 2 is a valid concern but GW is putting in a lot of effort to keeping the old relevant and there is big HH box set coming soon. It may happen but I wouldn't expect it for years.
As far as roles are concerned Firstborn and Primaris have units that fit in both categories. Your better off looking for the units you like, and then addressing the weak spots. If your not sure where to start, I'd look up battle reports on you tube, ask the scars players for advice, and if your planning on tournaments check out goon hammer (they have some great getting started articles).