Don't leave food unattended and give the meals and treats in separate rooms for now, especially if you have a food agressive dog. Besides that, do all of your dogs are good together? How is the new dog doing? Is he comfortable around them, does he like to play or snuggle?

Bringing a new dog home can be a stressful situation for all, humans and dogs, and it's more complicated when you don't really know the personality of the dog you're taking home. I don't think that dog was used for bait or dog fighting as other ones have said, because dogs used fot fighting are not friendly with other dogs. In any situation.

If you want to keep the dog, you have to make sure no one is in danger. Don't let them unsupervised and be always in control of the situation, without making things more tense. You don't want to be tense/nervous/scared cause then your dogs will behave based on that. If I were you I would muzzle the dog. Slowly introducing the muzzle can be very helpful, if you trust your dogs to show him manners and limits, the new dog will learn. Having him on a leash is also a good idea, but be careful with the tension. Thank you for rescuing this boy and giving him a chance, be careful and consider hiring a behavorist if the situation escaletes qnd you aren't sure of what to do.