Does your coop or run have an "apron" of hardware cloth around the perimeter to keep varmints from digging in? This should extend out about a foot around the coop or run on all sides and be securely fastened to your fencing, and be covered with a layer of dirt or gravel.
As LofMc said, a good, noisy dog helps deter squirrels. I have two Shelties but you have to be willing to put up with the barking, lol. I did not find them hard to train, I just used the "see-no-touch" method for a couple hours a day for a few weeks. Worked great and was easy.
A plastic owl, IME, is worthless. I've seen hordes of pigeons, at least, perching around and ON the one that was supposed to keep them away. Don't know if one would work on a hawk.
Have had no issues with cats including our own, but we keep them away from chicks under about 8 weeks old. Any younger than that and they look too much like baby birds and I would not trust a cat not to go after them. After 8 weeks they look a lot like chickens and we integrate them with the adult flock, so they're safe from the cats.
Hawks ... others will disagree with me, but ... what has worked for me is to have a few black hens in the flock. Crows will chase a hawk, and it's my belief that a hawk sees my black hens and decides to hunt elsewhere. I have not had losses to hawks since I got a few Black Australorps!
Finally, search "no waste feeders" here on the site and learn how to keep your feed from spilling on the ground and attracting varmints. Don't attract vermin with kitchen scraps. Have your compost far away from the coop. Lock up your garbage, feed your dog and cat IN the house. Good luck and enjoy your flock!