If you use layer feed for that young of birds the higher calcium level cause long term health problems. Can't remember if it's kidney or liver failure, but one of those.
All flock is s good alternative if you have them all together. Just be sure to provide free choice oyster shell for the layers, The youngs won't touch it until they are ready to lay. Stick with starter if they are in separate pens.
I never liked mash, it just seemed so wasteful. Pellets seemed big and crumbles is a happy medium (for me).