Why do you want to keep a rooster? The only reason you actually need a rooster is if you want fertile eggs. Everything else is just personal preference. Some people wouldn't have a flock without a rooster, they like them that much even if they don't hatch eggs. Some people wouldn't have a rooster with their fl0ock for any reason. The hens can be very happy both ways.
Is 9 hens too few to keep one rooster? Many breeders keep one rooster with one or two hens throughout the breeding season without the over-mating, barebacked, or stress problems. I've kept one rooster with as few as six and as many as nine without problems. Some people have had stressed out, over-mated, barebacked hens with one rooster and over 20 hens. Personality of the rooster and the hens (the hens paly a part in this) are much more important than the raw numbers.
I encourage people to keep as few roosters as they can. That's not because you are guaranteed problems with roosters, it's because the more roosters you have the more likely you are to have a problem. I agree you should get rid of two of those, the question is if the right number of roosters for you is 0 or 1.
Some people like to have a rooster around if they free range, they think they get protection from predators that way. In my experience you get some benefit, but it's not a lot. A rooster cannot (and most will not) stand up to most predators, things like dogs, foxes, coyotes, and many more. Some of them may attack a smaller hawk or something like that. To me the greatest benefit as far as protection is that a rooster will often investigate something suspicious, putting himself between the flock and the perceived threat until he decides if there is a danger or not. That means he is more likely to be the first to die. My roosters usually lead the flock toward safety once a threat is recognized than stay to defend the flock. I've had two dogs attacks with losses of 8 and then 5 chickens. In both cases the rooster survived unhurt. In most flocks if there is not a dominant rooster the dominant hen assumes many of the roles of a dominant rooster, including being an early warning system.
Some people are concerned that a rooster will become human aggressive. It can happen. I grew up on a farm with a free ranging flock that included roosters. A human aggressive rooster was never a problem and we were around that flock at a very early age. A young kid is often at a level where the claws, spurs, and beak can do some damage, especially to the eyes and face. Some people never have this kind of problem but there is a risk it could happen. If you have young kids you might want to consider it.
When chickens mate, the hen flattens herself on the ground, we call it the squat. That way the rooster's weight is transmitted through the hen's body into the ground instead of all that weight going through her legs. Roosters of the same breed are heavier than the hens, sometimes quite a bit heavier. This enables them to mate without harming the hen. Some flocks have full-sized roosters with bantams and never have problems with the rooster and bantams mating. But with living animals you don't get guarantees. The greater the weight difference the more likely there is to be a problem, especially if the hen resists and doesn't squat like she is supposed to.
It's normal behavior for a rooster to mate hens. To some people that appears to be pure brutality but to the chickens it's natural. Sometimes the hens cooperate, sometimes there is chasing and force involved. Normal rooster-hen interaction can really stress out some people, you see several posts on the forum about that.
But watching juveniles go through adolescence can really be hard on people. The cockerels normally mature earlier than the pullets. A standard way for a cockerel to establish dominance over his flock is to mate with the pullets. Often, or more like usually, the pullets don't accept his dominance or haven't matured enough to willingly squat so he chases them and forces them. As Sumi said, watching a flock of adolescents go through this phase is not for the faint of heart.
As a cockerel matures his behavior can change. The hormones don't always hit at the same time for all of them so hey mature at different rates. Also, if there Is a dominant cockerel or rooster in the flock his behavior can be suppressed. His personality can change dramatically when the dominant male is removed. I find that the later maturing or less dominant cockerels are more likely to be jerks later in the process than the ones that hit maturity early. But any of them can be "good" or any of them can be "bad", it's just the roll of the dice. I don't know what you mean by annoying and a jerk, but it's likely that cockerel and the others are just acting normally.
It's your choice of course, but from reading your post I'd think the correct number of roosters for you is probably zero. I think keeping one will stress you out, especially watching them go through adolescence.
