My dog ate some cooked squid that was mixed into a stew, is that dangerous?

No, it is not dangerous. Cooked plain squid is safe for dogs as long as it is plain and fully cooked. The meat is very chewy, so cut it into small pieces to avoid choking. Watch your dog for any signs of gagging or trouble swallowing, but serious problems are unlikely. If your pet shows persistent vomiting or unusual behavior, give your regular vet a call.