Table scraps should make up no more than 20% of a chooks daily intake.
A layer mash or pellet food is the way to go, this way they can less effectivly sort through the food. You can feed whole grains, but you will find that they will sort out the choicest morsels and leave things like sorghum. Ask they seller what they are feeding or recommend, this way there is less cahnge for them when you get them home.
Over about 12 weeks, a full sized (not bantam) chook will have little to fear from birds of prey or cats. A big eagle could in theory take one, but it would want to be pretty hungry and most chooks are pretty savvy to things overhead.
If you have foxes, do all you can to protect your birds, in the dead of winter, a sleeping chook is easy pickings for a mumma fox.
Younger chooks (teenagers) tend to be a bit flighty and twitty,the closer to laying they get, generally the more settled they get.
Backyard Poultry by Andy Vardy is a good place to find out heaps of info, go to the info centre top right of screen. (and you may just see the odd BBer there too!)