I will say it until i die TRAIN TRAIN TRAIN. Not by shock collars. You could use a click trick though no pain no loud noises. Just clicks!
Anyways, some dogs just like humans tend to go a little off their rocker when they get older. But constant training will be effective. Make sure it is constant! Different types of animals living together can be possible with no extra stress. I mean how many people thought dogs and cats couldn't live together.
I do know what you mean when you say separation is not a possibility. It isn't in my house either. We are all equal and all a family. I could never make my dogs stay in "their own room." That sounds awful for them. And as for baby gates, well that would block off half he house for them to roam and run and play with toys and to sleep, I could never do it. You think piggies need a lot of space in their cage think of how much bigger your dog is to your pigs and how much space they really need to be happy and healthy. As for cages and small animals it is humane for the pure fact that so many things outside their cage could harm them. Plus their poo wouldn't big so great all over the floor.
Having a baby gate at first to train your dog back into shape is good! But you can't keep that up forever. Training will work as long as you are diligent!