(sigh).... Ruthie sounds a lot like Banks. Hit or miss depending on the other dog and her mood.
Banks does great with my sisters dog when we are at my sisters house, bring Pugsley over here and it is a differently story. Banks also HATES dogs with long hair Newfoundlands are her ultimate enemy - she just hates them. One of the big things YOU need to do is control your thoughts and feeling on how she may or may not react because if she senses your insecurity of the situation. Mind you, I know all this now after having Banks in training for 8 yrs. there are just certain situations we know just will not in anyway work for her and knowing that, we realize there will never be another dog into the home unless it is a puppy, as that is all she will accept.
Banks is very dominate and it is just her world that she allows us to participate in. We make it work with Cheli and she has come to accept him as part of her pack, but she also has her days when she will attack him for just entering the room.... she is just that beeeotchy. Cheli was 8 months old when we got him and we spent a full week of off-site neutral ground introducing them and getting her to accept him.... introducing on neutral ground is beyond HUGH... take them for walks to get them to pack up.... it is a long journey and can be done, but it is a long journey that will have set backs here and there.
@brutus77 just added to her home she may be able to give some more insight
good luck and try to relax so Ruthie will be relaxed