Since you adopted him from a shelter you do not know of his full past history. It is possible that he was teased with treats before and now has become very territorial with them and afraid someone is going to take it away from him. It is also possible that he was mistreated before and that is the cause for his snapping at your daughter and husband.Sucking on items and even carpet canbe common with dogs and it can have to do with the nutrients in their diet that they may not be getting as well as with him it could be a comforting action since he has been acting so strangly. This can also occur when they have been taken or lost their mother too soon.
I would recommend that you make sure to feed him a quality dog food from a pet store that has all the needed nutrients and that may help with his suckng on the animal. EaglePack has a wonderful line of foods and that is what I would recommend that you feed him.
Also they do sell an all natural remedy for stress and anxiety at most all pet stores and I feel you should try this on him to see if it will help with the way he is acting. You can mix it in with his food or gove it to him directly and I feel that would help with his actions. If he continues to snap at other dogs and display aggressive behavior you might want to consider having a trainer come in and work with him since you do not know of his past and if he was abused in anyway it would account for this behavior but I would try the dg food and the anti-anxiety medication first and see if they do not help.But do still keep your girls away from him when he is eating any treats as it is very common for dogs to be very territorial when they have treats.Also if he continues to get that far away look in his eyes I would recommend that you have some bloodwork performed on him to make sure there are no underlying health issues. Please do let me know if you have any further questions.