Getting dogs to accept each other

Background, we have three dogs currently, two pit bull mixes and a chihuahua mix. The chihuahua runs things. The hundred pound pit bull runs around with the chihuahua. The 50 lb. pit bull lives on my bed. She has issues, she's a rescue and is afraid of everything, and when she is afraid, she is aggressive. We have had her for about 1 1/2-2 years, she now spends a couple hours on a leash around the other dogs. I know that I'm going to have to drop the leash and just let them work it out at some point, but what would be a good signal that that time has come.

Further background, we had a dog kill another dog about 10 years ago, and would not like to repeat that scenario.

Evaluating the situation sight unseen is totally impossible. I'm sorry. But I'd like to make a few points, which you may already be aware of. I'm stating them to help others who may be considering taking on a similar problem.

...You have a big dog/little dog situation, where the big dog has aggressive tendencies. Sadly this is a common enough scenario that veterinarians and dog parks are well aware of the deadly consequences. The little dog is frequently killed or horrendously damaged. People are often bitten or savaged while trying to rescue the little dog during the attack.
...You are working with breeds known to have what breeders refer to as a "fighting mentality". It's as though they have switch inside their head that flicks into attack mode, but once the fight is over, the switch goes back to normal friendly dog. That's why owners will swear that their pit bull is a sweetie and are aghast to see it turn into a killer for a few minutes, then back to a sweetie. Owners have actually been killed by their loving dog when it clicked into attack mode and they were its unlucky target. Pit bulls are not the only fighting breed out there that exhibits this trait. I had Shiba Inu for decades and was well aware of this trait in that breed, but luckily my dogs were in the 15 to 20 lb range and thus manageable. Most breeds bred for fighting inherit this trait, though surely not all individuals have it. Show oriented breeders tend to selectively breed away from it.
...Your rescue dog has significant personality issues. While some may have been acquired, some may also have been inherited. Regardless, they aren't going away anytime soon since you've had the dog for a long time and the issues still exist.

I would suggest having the situation evaluated by a canine behaviorist or several experienced dog trainers. Seeing the situation firsthand, they may be able to offer suggestions. Just dropping the leash and letting them work it out is not a good suggestion in my opinion.

@Su Ba: "I would suggest having the situation evaluated by a canine behaviorist or several experienced dog trainers. Seeing the situation firsthand, they may be able to offer suggestions. Just dropping the leash and letting them work it out is not a good suggestion in my opinion."

My wife was nodding her head in agreement with this advice Su Ba,.....how timely.

Until last Thursday, we had 5 dogs including a small rescue that was a lap dog.   All dogs are allowed freely into and out of the house and have their "spaces" and the little lap dog, certainly aging and losing some of her astuteness, had her space next to me on the chair and in the bed.  Three of the dogs are Anatolian's and spend most of the time outside.....you know where this is going.  Last Thursday during normal chores, the dogs were playing around as usual while I was distributing water, chicken feed, etc.  For some reason they all tried to follow me through a gate and the little dog tried to dart between the legs of the LGDs to get to me.....a fatal mistake.  By the time I spun around from her yelps, it was probably too late......as you say, two of the LGDs had switched into killer mode.  Yeah, I'm big and dumb.....I jumped in between them trying to shield her from them.  And no, they weren't going to stop on account of my screams to do so. When I emerged from the fray, little lap dog was not dead, but too far gone.  Covered in blood, I held her for the 10 minutes as she passed over.  The big dogs?...."Fight???....what fight???"  Like nothing had ever happened and they are ready to be friends again.....which we have not rekindled yet as we plan our behavioral strategy.  Don't know exactly what that will be, but for the time being they are banished to the outside (where LGDs probably should always be anyway).  As my wife said, if they had meant to harm me in this scuffle, I would have known it.

Anyway, one more story, and a good discussion on being aware of your dogs cues and the hierarchy of the homestead.

Cannot agree enough with the other responders on this thread. These situations become dangerous because there is an artificial separation in the house- while everyone lives there, there are not definite rules understood by everyone as to who goes where and does what. If you (or, again, an experience behaviorist) doesn't sort these things out and establish ground rules, the dogs will do so themselves, and a 50lb pitbull doesn't understand that she can't censure a chihuahua the same way she would another pitbull. That's even before the breed issue, as Su Ba rightly points out, of the attack instinct. Dogs are very social creatures, and they will ultimately find an order and a situation that works for them.

There are common ways to help establish these boundaries and relationships, and many books cover this subject. Shared tasks are a common way, particularly for dogs bred to tasks like pitbulls (terrier), to establish a 'working relationship'. Even something as simple as walking dogs together can go a long way to establish this relationship, (highly recommend MORE PEOPLE THAN DOGS when doing so). Some people advocate taking an active role in the establishment of the pack hierarchy through things like controlling who eats first, who goes through doors first, etc. This is probably not as feasible in your case, because whatever relationship they have is not new ground. In your case I think the opposite makes more sense- I would separate them for meals, prevent circumstances where everyone is dashing through the door at once, do everything to limit those moments when they enforce a hierarchy themselves.

@John I'm so sorry for your loss. Losing a dog is heartbreaking, losing a dog suddenly is even worse.