I have experienced lots of litter mates being raised together and there has never ever been any problems. Also as far as the neutered dogs go I have never experienced any major issues with them with entire males.
Im commenting on large groups of kenneled working dogs, who mix together when they are out. All the dogs regardless of whether they are neutered or not, soon learn their place within the pack as if they do start to become cockie the top dogs put them in their place. Its all about respect and its the same in the home, once a dog has learned respect of others they soon know how to behave. Also it a good way for a young dog to learn not to be so dam arrogant and forward and makes them think twice about breanging in.
Some dogs regardless of sex or entirety, will challenge other dogs when on a lead it is a defence mechanism, but let most dogs off the lead together and they will soon start to enjoy each others company (unless its a blood thirsty beast).
The so called behaviour person, well Id dump them for a start and find someone else, how dare they suggest you rehome one of you're dogs
I really wish you were nearer as I would love to help you with this. I really would suggest to work hard on the socialisation and hopefully someone with experience can move things on for you.