That’s a shame. is it a Kill shelter?
Your parents will say, “No!” at first, so be prepared to talk like a salesman to overcome their objections. Or beg them to just keep the dog for two nights before you bring her back.
If you want to give it another try, i’d be willing to be your online “buddy” and provide an all-day Sounding Board and answer questions to the best of my ability, to ease the transition. The first days ARE difficult.
I hope you’re able to save another pooch. Well Done.
My old dog who I recently put to sleep couldn’t really sit. She could squat uncomfortably, or she did this weird looking thing where she laid one side of her pelvis on the ground and stuck her hind legs forward between her front while somehow keeping her body upright. She was a Tibetan Terrier and had such long legs she looked like a little pony, I always assumed that was why (there are some things I can’t do because I am lanky, after all).
This thread made my day and I look forward to hearing more about Sweet Pickles.
Oh also my remaining dog is insecure and neurotic, and before my first dog got senile she was a great companion for him. So I think the ‘pet for my pet’ idea has some validity. Just has to be the right pet.
It’s a very nice no-kill shelter and she has a fan club there, so I’m sure she’ll do fine. I should have known better than to get the dramatically ugly dog anyway - it’s the humdrum lab and pit mixes that don’t get adopted. The three legged zombie eye dogs get snapped up.
I’m just really heartbroken about the whole thing.
Dogs are individuals, and sometimes two individuals do not manage to hit it off. You have certainly given it the old college try. Here’s hoping Sweet Pickles gets a situation that suits her better.
Would it be possible to introduce a potential partner to Captain before committing to adopting? Have them meet on neutral ground (neither dog’s territory?)
This is a really good point. Will the shelter allow you to bring Cappy with you when you go there? Let him pick his buddy. . .
I don’t know if it would help at all - they were fine when we first introduced them (well, she was kind of frozen, but no snapping.) It’s only the next day that the Great War begun. And poor Captain is so sensitive and submissive that he didn’t fight back, just screamed and got his face ripped up. 
Definitely we couldn’t just take him to the shelter - he’d be so freaked out by it that I’m not sure we’d get anything useful out of it. At any rate, we won’t be considering it for a little while. I don’t even know if I can go in to take her back - if they don’t need a signature or anything I might make my husband do it.
Oh jeez, I have introduced a ton of dogs, many of them bad at introductions, and there has never been a serious (as in, bleeding) bite to the face involved. Poor old Captain! He is your first priority, I’m sure Pickles will find another foster family soon (and it sounds like a tank such as her won’t find the shelter environment particularly stressful). I’m sorry you’re feeling guilty/sad, try not to. 
You sound like such a great dog owner. I hope you’re able to find a more compatible foster that will be a good friend to Captain at some point.
For the record there is no forcing in my method. If you push gently on the back of the legs (I use the term “karate chop” to describe how to place your hand) and pull up with the leash the dog’s body will end up in the sit position. No forcing necessary. Tons less physical exertion between bodies than what the OP was trying to do - push the dog’s butt down (which is a natural thing to try and no doubt I tried it too before a trainer showed me this other method).
The point is moot now for the OP but it’s still a valid way to get a dog to bend their legs and put their butt on the floor, and in no way is forceful or mean.
no, there’s no way a dog can “sit” as in meditation; to paraphrase a Tibetan monk, if all the Buddha’s that have ever been came before your pet and personally talked with your pet for 30 days, at the end of that period all your dog/cat (etc) would do is wag its tail, hoping to get some food. you might want to think about that the next time you seek a level of relationship from your pet that it’s incapable of providing. don’t blame your pet - it has nothing but love - when you seek from it what it can not give, namely human companionship…
People who say this kind of thing aren’t very close observers of animals. I’ve known dogs who would much rather interact with their people than eat.