Will it be ok to fed this?

It's the calcium % is what your concerned about. I have read though, too much protien too fast growth rate in bones that could lead to leg problems. I go 20% starter, 15% grower/finisher, 18% Layer, some folks go with a 20% layer.

i would feed it....in fact i have

They will do great on the Flock Raiser. Just remember to provide oyster shell to the girls that are laying age.

I've been using Flock Raiser almost from day one because it is the only kind you can buy here that isn't medicated, unless you want to buy 5 lb. bags I will switch to a layer feed when they are old enough.

Many feed well over 20% protein.  I don't think you need to worry about too much protein causing a problem until you get near 30% and even then a few people on here feed 28%.  If you aren't using layer feed though you should provide some other source of calcium like oyster shell or crushed up egg shells can help.  Since it's a good idea to put out oyster shell just in case even if you are feeding layer it takes no more effort for me.  I use 22% game bird feed for everyone so I don't have to worry about age groups.

ive read that the extra protein is good for them because unlike the factory chickens, backyard chickens are outside running after bugs and getting exercise

I'd buy it, but wait to put it out free choice until, #1 - some are 20 weeks old, OR #2 - you get your first egg, whichever occurs first.

I put crushed oyster shell in an empty tuna can nailed to an inside wall of the coop, near the feeder.  I keep it full.   And I feed all different ages the same feed, grower/finisher (equivalent to FlockRaiser) all the time.   Well, chicks under 8 weeks are kept separately and fed chick starter....   But I have two ducks as well as four different age groups in my flock.

i use flock raiser , everything els in the store i go to is medicated , they do fine with it