I wouldn't get too hung up on all this. You can feed chick starter all the way through. Just mix it 50-50 with oats. But starter does not come in pellets (usually), which means waste and extra expense. Many times the feed store will only have chick starter and layer pellets. For a small mixed flock between 3-6 months, you can go either way, starter mix with oats or layer feed. I don't believe the concerns about calcium really apply to birds of this age (you could mix it with oats and cut down on the calcium). I don't believe added calcium will make them develop more or less quickly.
Excess protein can be a problem. So, if you can't mix with oats, for birds of that age, I think you are better off with a 14-16% layer than a 18-22% starter.
But many feed stores will have something else. Like I said, it might be labeled for turkeys. Check the tags. If the protein is 14-16%, and the calcium is <3%, it would be perfect for this age group. You could feed that for the rest of their little chicken lives if you set out oyster shell along with it.