these are Easter Eggers so I'm not sure there are any "standards".
EE's are not a breed so no, there are no breed standards. We can't even all agree on what makes an EE. Why are you calling them EE's?
My goal is to pick a breeding rooster that will throw the blue egg gene.
This goes back to my other question, why are you calling them EE's. How do you know or why do you think any of them might have the blue egg gene? That might play into which one to keep, depending on your answer.
* do I pick an alpha or one of the more timid roos?
With my goals and how I keep them I pick an early maturing one. That is not necessarily the Alpha but is usually one near the top of the pecking order. Part of that is that I raise them to eat, so I want an early maturing one because they are typically ready to eat (bigger) earlier so I don't have to spend as much on feed. I want a rooster that has the self-confidence and personality to win the girls hearts by the strength of his personality, not purely by brute strength. I don't always get it right but I've had more luck with an earlier maturing one.
*is there a particular colouring combination that is better for breeding EEs?
Absolutely. What color combination do you personally like? That's the best color/pattern for you. I'm sure my personal preference is different, so that determines the best for me. But that doesn't mean anything to you, it's your preference that counts. Of course there can be exceptions, there always are, but black or white roosters tend to have mostly black or white chicks. The color of the hen seems to have more of an influee with a red or buff rooster.
*there is one rooster that seems bonded to a group of the younger girls. Is that a good sign?
Maybe. I don't now how you manage them, how much room they have, how old they are (Spring is kind of nebulous), or how they are behaving. When you have more than one they can influence each other's behaviors. Those might change when you process the others.
*are their certain behaviours I should look for that indicates a better rooster?
With more than one around they can influence each other's behaviors, especially with the girls. They are probably still adolescent's too. Immature cockerels have different behaviors than mature roosters, behaviors can change as they mature. It is a challenge when they are still immature but I also make those decisions fairly early.
If one is human aggressive he is gone. If one injures a pullet he is gone. I do not consider an immature cockerel forcefully mating an unwilling pullet as injuring her. To me that is just what immature cockerels hopped up on hormones do. But if blood is drawn or he looks like he is trying to injure her it is frowned upon. Severely.
My main suggestion is to decide what your goals and preferences are. Not mine or anyone else's, but yours. Then start eliminating those that obviously don't measure up. When that decision starts getting hard you may be OK with any of them. I once did that with 18 cockerels. The first 15 weren't all that hard, the last three were rough.