Yes, they know when to stop eating.
The thin one might need to eat extra food for a while, to gain weight again.
The ones at a healthy weight might seem to pig out on the mush, but then they will eat less of the dry food, so they will still be eating the right total amount each day.
You might try adding more feeders & waterers to their pen. Or, if you already have multiple feeders, add something so the chickens can not always see who is eating when. That way the lower ranking birds can eat without being harassed.
Of course you can give them mush as often as you like, too.
I do not know why chickens like wet chicken food so much better than dry, but they sure do!
