I would agree that you need to judge the temperament of your dogs with small animals. You know your dogs. If you think your dogs would be ok with a cat, then you should adopt either an older cat that knows about dogs, or a kitten that won't get so rattled....older cats tend to act out when put into new stressful situations like that. My old girl used to use anything that smelled like me to poop and pee on when she was unhappy with life, regardless how diligent I was about cleaning her litter.
No, it is not 'wrong' or 'bad' to put the cat into a separate room while you are away, although if you only had one cat, they might get lonely. If you got two or more, go for it. Give them litter, food, toys, beds, etc, and close the door. They will be just fine.
I also wouldn't worry about the cat and dogs working out the dynamics. The cat will rule in most cases, unless you have one of the dogs 'adopt' a kitten, or a really alpha dog (in which case, get a kitten or prepare for WWIII).
If you have managed to train two newfs, training a cat will be easy.
1) they want to use litter, it is natural for them to scratch once they can feel the need to go on their own (like a dog, the mum stimulates their bowel movements when they are very small)
2) Like a dog, they will get into everything and go through teething stages as kittens. Unlike a dog, they won't likely chew through your leather shoes and underpants that are in the hamper. get some tantalizing toys with some cat nip, and you'll be good. They will investigate everything...no plant is safe. not even hanging ones, kittens are like flying squirrels. get rid of poisonous ones.
3)My Trina liked to chew wires as a kitten. I ended up covering them in cheep chap stick (go figure, she liked the bitter lime stuff, chap stick makes her freak and act like she's got a tick)
4)My cats are corrected from bad behavior with a sharp clap from my hands, or a squirt from a water bottle...Trina adapted to the water, so she got vinegar for a while..now just a shake of the bottle and she stops. Lulu responds to hands...she has yet to be squirted (she's only 15 wks)
5) either declaw your cat or learn the art of trimming nails. It isn't hard usually. neither of my current cats are declawed and my furniture has survived. Invest in a maraud of scratching items. I have a wood post, carpet 'tree', and a post wrapped in twine....they use all 3 regularly and leave my furniture alone. Trina was easy to train, again this is natural for them. I just took her over and massaged her paws against the post and she took too it, was told she was a good girl and got treats. Same with Lulu, but she has her sister to imitate. they get sprayed if caught in the act of scratching a bad spot, then taken over to the right spot and I do the massage thing.
I've rescued my old girl Gabby at 11, my she-devil Trina at 6mos, and my baby girl LuLu at 12 wks. Both Lulu and Trina are affectionate, Trina is fine with dogs as long as they play with her and don't upset the kitten too much, Lulu still walks sideways around most dogs and pitches a fit, but she is getting better (don't stress about poofy tails, hissing, arched backs, and batting...that doesn't harm anyone...the dogs may get 'tuned' until rolls are established, then it will calm down). Gabby was fine with dogs as long as they left her alone, otherwise they got tuned...she was the boss. Trina went wacko on my friends pug mix that tried to mouth Lulu a few weeks ago...ripped out a few nails and sent Jessie running for the hills. No one messes with her baby girl I guess, she's a momma cat now. However, no one was seriously hurt and Jessie gives both cats space now...no more mouthing the itteh bitteh kitteh, momma cat knows where the gonads are!