A good rescue group and a responsible breeder usually have the same requirements for placing their dogs, so those requirements shouldn't be a factor in your decision of which to get.
Not all male Dobermans are male dog aggressiv e but most of them are, so imo it's not worth the risk to take the chance of fighting. it doesn't matter if the beagle puppy is submissive or not, when both dogs reach adulthood is when the fightin would start, and when he is an adult the beagle may not be so submissive any more anyway.
Personally i wouldn't place either a puppy or a rescue dog with someone who doesn't have a fence, no matter where they live. you don't have to fence the entire property, just enough to provide a safe place for the dog to go to relieve itself and get a little esxercise.