When taking into account that Dodger is named after Jack "Artful Dodger" Dawkins from Oliver Twist and that he's a terrier in the film, you'll realize that he's a Jack Russel Terrier.
Why are the Dobermans easily subdued with a blanket? One way to test a dog's intelligence is to throw a blanket over their head ands how long it takes for them to shake it off. Considering Roscoe and De Soto are considered unintelligent, it makes sense that they wouldn't try to shake the blanket off of them.
At first, this brings up some Fridge Logic: Dobermans are an intelligent breed of dogs. But then, it dawns on you: Dobermans are supposed to be gentle too. The only problem is, that all depends on how they're brought up. And don't forget, Sykes is their master. If he can bring up Roscoe and Desoto to be sadistic, even for dogs, then it's possible he neglected to exercise their minds (aside from being thugs).
It just came to me: Old Luey is the movie's equivalent of the food server from the book who was outraged when Oliver pleaded "Please sir, may I have some more?"
We see that Sykes has a gun, so why does he use attack dogs? Body disposal. If there's nothing left but a few gnawed bones there's a lot less for the police to go on, if they investigate at all.
He snaps his fingers a second time to call the dogs off. Them ripping into you is another form of persuasion.
When Sykes sics his dogs on Fagin and Dodger, he nonchalantly goes back to building his model ship like nothing's happening. He's likely done this kind of thing so many times he's numb to the violence.
While it's hard to tell if it's brought up in the film, older viewers will get chills when they realize that Sykes has a multitude of ways to make money off of Jenny if her parents don't pay up and none of them would end well for her. Doesn't help that she's extremely adorable either.