I have a 5 month old brahma cockerel, a 15 week old cockerel, and lots of little 9 week old cockerels
They are all too immature to challenge the rooster. Until they mature enough to stand up to him they are going to run from him. Even if the dominant rooster were not around they are too immature to know how they will eventually act with the hens when they grow up. When the hormones hit behaviors can get pretty wild but when they mature enough to get those hormones under control they can really mellow out. And the girls are much more likely to accept them.
The brahma does, however, drop his shoulder a lot and circle me....doesn't seem promising.
This sounds like the hormones have hit him and he thinks you may be subordinate to him. This doesn't mean he is going to become human aggressive but when he does this I'd calmly walk toward him and cause him to walk away. Let him know you are not intimidated or afraid of him. He may consider this a challenge and attack which I'd consider a sign it was headed somewhere not good or he may understand you re not to be messed with.
Which one do you think would be my best bet for a good rooster?
What are your goals? What do you want him for? If you just want a spare that could be any of them. If you want him for breeding what traits do you want him to pass down to his children? Something to do with his appearance like colors, patterns, comb type, size? It could be the color of eggs his daughters might lay. Is he of a breed that the hens typically lay a lot of eggs? If it is behaviors, that can be hard as long as there is a dominant chicken around. Even a dominant hen can suppress the good or bad behaviors of an immature cockerel until he matures enough to take over. This tendency of immature cockerels to have their behaviors suppressed by more mature chickens makes it hard. The changes that the hormones make can also make it hard to g by behaviors.
The way I approach it is to know my goals and what I'm looking for and start removing any that don't match up. This narrows the numbers down. By the time I get to the last two or three they are all usually a pretty good choice, that's when it gets hard.
I have 18 pullets right now and they free range on over an acre. Is that enough for two roosters?
Some people really truly believe that the ratio of girls to buys matter. They may have personal experience to believe that way. I don't. One of my most peaceful multiple rooster flocks was 15 hens and 3 roosters. They free ranged and got along great. I once had a peaceful flock of 1 boy and 8 girls. Two of the girls became barebacked so I ate the two girls. That left me with 1 boy and 6 girls. The were still peaceful and the barebacked issue went away. Dad's typical flock was one rooster and 25 to 30 hens. I personally don't consider the boy to girl ratio to be that important.
I don't think you'll get much argument that 18 girls is plenty for two boys.