ARRRRGH!!!
Bloody ignoramuses!!
Sorry - it's just that I'm both an independent rescuer and a volunteer for a rescue shelter and hear this kind of crap all the time.
All too often there is one and only one reason why such a dog is in rescue... and that is because of scare-mongering, ignorant opinions which are bandied around by people like your PIL. If I had a pound for every person who came up with this crap or a pound for every dog which was languishing in rescue for years because of it, I would be able to afford to save them all.
Rotts are right up there with Staffies as allegedly dangerous dogs by those who know no better and GSD are not far behind. Which, as you gathered, really pisses me off, particularly as I own 2 GSDs (and a Labrador and 2 daughters!) and as my daughters are also rescue volunteers, one of whom spends a particular amount of time with Danny, her favourite of the 60 dogs in our local rescue. Danny happens to be a Rottie.
No particular breed is a danger. That's as logical as saying that if you come from Mexico you're a danger but if you come from Tunisia you're fine. Daft, eh?
Tell me, what have the rescue told you about the dog you've fallen for? I'd think, unless they are a dodgy one - and I don't deny they exist, not all rescues are as scrupulous as the one I volunteer for or those I work alongside - they probably know the dog extremely bloody well after 6 years!!
If you have had a homecheck and been approved, if you have had long and earnest, honest conversations with the rescue, if you and all your household have met and interacted with this dog under the supervision of the rescue, and if this rescue is reputable, I think you have no reason to be suspicious. If these things aren't insisted upon then you must think again.
You know what you want and need in a dog. Now what you require is to know what you need in a rescue. If they have homechecked/will homecheck you, if they have vaccinated, neutered and microchipped the dog, if they have assessed you both, met the household and give you a written contract promising you lifetime support and that they will take the dog back at any stage in his life if you are unable to keep him, you are fine. If you are not given these things, consider VERY carefully whether you want to proceed. At the very least. Better still, walk away, there are thousands of other dogs begging for homes and hundreds of rescues which WILL offer these things... and I would consider these things to be essential for those with a young family and without rescue contacts.
Finally, may I ask which rescue it is you are dealing with? I may have more knowledge of and/or info on them or know someone who does.