ok, so let's say bullseye is attacking a couple skrulls, a friendly doombot changes to shield to boost B'eye's damage to 2 for each skrull. Bullseye rolls the attack and makes it for both, then the skrulls both roll their team ability and make it. THen the questions started arising.
First of all, does skrull need to be rolled before the attack roll?
Next, is shield paid for before or when the attack roll occurs? This means that i already put a token on Doomb. but was i ahead of myself? If BE was not able to attack the skrulls did the shield team ever actually happen?
Next q. If everything was fine so far, could he cross his guns to target the opposite skrull with a "different" arrow, allowing them to roll their team abilities again?
Finally, if there was no way he could attack those skrulls again, what actions could bullseye or doombot still take?

thanks in advance.

I believe this is stated n the FAQ. The Skrull role should happen before the attack role is made. Then Bullseye is free to attack whomever else he wishes. The second Skrull would also get to make his saving roll.

I'm not sure about the Shield boys though. My guess would be that since you can't take back the movement part of a running shot tha the Shield boys would also be unable to take back their move action which allows them to add the +1. Of course, a Judge may have a different take..Let's see.

in reference to bullseye, is it ok for him to take a move action after the skrull is resolved if there are no other targets? or could he be taxi'ed then take another attack?

The skrull roll should be made when the targets are declared and the attacker is checking to see if they have clear LOF to the target. A successful skrull makes it so that fig is treated as a friendly fig and friendly figs are not valid targets for a targetted attack, so you can't make the attack.

The shield team ability happens as part of the ranged attack action. Since you couldn't make a ranged attack the shield ability isn't used and no token is placed on the shield figure.

your question about choosing "different" targets is somewhat tricky, mostly because the skrull power is poorly defined. Your example of BE "crossing his guns" certainly doesn't work, there is no mechanical difference from the attack you declared (it would just be a cheesy way to try to get around the ability). From the wording in the faq and PAC you could choose a different attack type against the skrull, such as a b2b hovering character's range attack being foiled by the skrull ability and then declaring a close combat attack (which would force a new skrull roll, also cheesy, by appears to be allowed by the rules).

You couldn't probably even make a decent argument for the case where the b2b hovering ranged attacker has the close combat attack foiled by the skrull ability as well, which would allow a new declaration of a ranged attack action (and another skrull roll). I would hope that your judge would rule against this, but it isn't clear in the rules.

Hopefully the skrull ability will be more clearly defined someday, cleaning up little problems like this.

The simple answer to your question about what BE can do next is: any legal action he can take except for the action he just attempted against the skrulls.

Hope this helps