I've had multiple dogs for decades (up to four at a time, not counting foster dogs) but have never had all one sex pack. Mostly Rottweilers, and I was careful about adding dogs so the "power structure" would stay balanced. I did live for three years with two male Rottweilers who had to be kept separate at all times. Both absolutely fantastic and very well-trained dogs, but they just would not tolerate each other. That was a pain in the ass and I never want to do that again! Both were very dominant and opinionated, and I did think about rehoming one but finding a suitable owner would have been a challenge. So we made it work but it sucked.
My best friend has three males - one a Rottweiler/Chow, the other two Chow/Lab litter mates. She maintains a firm hand but does have to watch them and the litter mates are separated from the Rott/Chow when they leave the house because there have been incidents.
With your youngest only being 9 months and still awaiting final brain shipments
, my opinion is to not add another boy. As stated above, may go great for life, but if it doesn't then you're stuck with a difficult situation.