Personally I don’t believe in magic numbers for chickens. We keep them in so many different situations and conditions, for so many different goals, and they have such different personalities, there is no magic ratio that solves all problems. I think how much room they have is a real critical factor. I’ve had just as many problems with a real good hen to rooster ratio as when I had a ratio that people would call bad. Each flock is unique. Still, I always recommend that 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 problems are more likely.
In your situation, with that small coop and what sounds like somewhat restricted run area at least part of the time, I’d strongly suggest no more than one rooster maximum. You simply don’t have the room for more than that. You might follow the link in my signature for some of my thoughts on room before you get more hens. The only reason you need a rooster is if you want fertile eggs. Everything else is personal preference. The hens will be quite happy without a rooster and will lay just as well. If eggs is all you want then zero roosters might be the right number for you.
Those chicks may have hatched from Silkie eggs. The gene that gives the Silkie feathering is recessive, it has to pair up at that gene pair for the feathering to show up. If the rooster that fertilized those eggs was not a Silkie, the Silkie feathering would not show.
Breed doesn’t really matter when you are looking for more hens. If you are looking for eggs only, just get hens that should lay a lot of eggs. They’ll fit in and make a nice flock. Size of the hen doesn’t matter that much and I’ve always kept chickens of various colors together without any problems from that.
If you can, try to get hens about the same age as yours. Integrating new chickens can get tricky, especially in small spaces. The more room you can give them, the easier it usually goes. Maturity is very important in that too. They will have to establish a pecking order when you put them together. More mature chickens outrank immature ones and can sometimes be quite brutal if the more immature cannot get away. What often happens when two chicken that don’t know where they rank in the pecking order share personal space is that one pecks or tries to intimidate the other. If one chicken runs away they have determined pecking order, though there may be some chasing and running away, plus some repeat performances. If one does not run away there will almost certainly be a fight, but usually it’s not long before one decides it’s better to run away.
Even if they have settled the pecking order, if the weaker invades the personal space of the stronger there can still be pecking and chasing. That’s why you often see younger chickens form their own separate flock when you integrate them, they just try to stay away from the older chickens. It helps tremendously to have enough room for them to run away or avoid.
Sometimes integration goes so smoothly that you wonder what all these predictions of gloom and doom are all about. Sometimes they end in disaster. Most of the time there is some bullying and skirmishing but they work it out. Many of us do that all the time quite successfully, but you can read some horror stories on this forum about that. They are living animals, no one can tell you what will happen in your unique situation.
If you do get more hens that are younger or older than your current hen, you can still integrate them, it is just a little harder.
The sooner you take care of the extra cockerels the better. At three months they are getting ready to hit puberty. The hormones are very soon going to hit them really hard. The cockerels will start fighting for flock dominance. If the loser in these fights has enough room to run away these usually end OK though they can get pretty vicious. Sometimes even with room injuries or death can happen. I have lots of room. I had one cockerel dislocate a leg this year in these fights. When the others sensed that weakness they tried to kill it. It could not run away. If you don’t have any females with the cockerels these fights are usually not nearly as bad, usually no worse than a flock of pure hens determining pecking order, but if a female is present those hormones take strong control.
They will also start mating with that pullet. She won’t know what is going on because the cockerels will mature before she does so she will probably resist. They will force her, especially if she can’t run away due to restricted space, though with that kind of ratio they will catch her anyway. It will almost certainly be very hard on her and she could get injured. You may be better off separating the pullet from the others if you can’t get rid of the extra cockerels really soon.
Good luck with this. You can do this quite successfully. Many of us do regularly.