Sorry- didn't look for replies until now.
Extra calcium for any prolonged period before the 16 week age-range can cause kidney problems, bone growth issues and possibly oviduct problems down the line- that's why foods like flock raiser aren't adequate sources of calcium by themselves- you ought not give that high calcium to chicks.
Each of the different feeds is formulated to be the minimum to meet the needs of a particular age range, and flock raisers are meant to meet the minimums of multiple ages with you providing supplementation in the form of oyster shell for the adults.
It really important to follow certain guidelines for protein and nutritional needs for various ages.
Some guidelines:
Approximate Ages (weeks) Crude protein % Calcium % Phosphorus %
Starter 0 - 6 weeks 18 0.9 0.45
Grower 6 - 16 weeks 16 0.8 0.45
Pre-layer 16 - 19 weeks 17 2.0 0.45
Laying > 19 17 - 19 3.8 - 4.0 0.45
You'll see that many commercial feeds targeting a specific age are close to, but not exactly at, these levels, so it's just what you find works for you- but it's important to note that you'll need higher protein when they are recovering from a molt. It's also important to note that scratch is only 8% protein, so every bit you give them decreases their net protein intake- it's chicken Cap'n Crunch.
It's no big deal to change them around for a few days, but you'll want to stick to guidelines for any lengthy duration.
For mixed age groups, a flock raiser is the best choice, and just offer crushed oyster shell AND grit for everyone to take as they please.