I always recommend you keep as few roosters as you can and still meet your goals. It’s not that you are guaranteed problems with more roosters, just that it is more likely to have problems if you have more, especially the way a lot of people on this forum keep chickens. The only reason you need a rooster is if you want fertile eggs. Everything else is personal preference. For a lot of people on this forum, especially with small backyard flocks in limited space, the best number of roosters to keep is zero.
I don’t much believe in magic numbers for chickens. That means square feet per chicken, how many inches of roost space per chicken, a specific age to integrate, or a hen to rooster ratio. We keep them in such different conditions, with different flock make-ups, with different goals, in different climates, keep different breeds or mixes, and have our own experiences that there cannot be one magic number that works for everyone. I have no doubt that Free Feathers can and does successfully keep chickens with that low a hen to rooster ratio but I’d venture that those chickens have a lot more room that a typical backyard flock. Different conditions allow you to keep them different ways.
That hen will be OK by herself. Keep the chicks near her if you can so they get used to each other but safely seaprated. She will probably want to get to those chicks because they are flock animals, but just because she wants the company doesn’t mean she isn’t a threat to them. Many of us integrate pretty young chicks with the flock without issue, but each flock has its own dynamics and we all have different conditions. The more room they have the better. I don’t know the right age for you to integrate them. That’s going to depend on your set-up and the personality of your chickens, especially the hen. When you do integrate, do it at a time you can watch and take action if you need to.
Good luck!