1. Diamond Formation is being enact before combat. Would the characters need to maintain the Diamond shape stand for the buff to be active?
The effect of moving in Diamond Formation seem to be that they get a bonus on perception tests against ambush attempts and a +2 initiative score when combat starts. Once members are not actually moving in Diamond Formation it seem as if they will no longer gain the benefit of the Diamond Formation maneuver.
For example, if the squad arrive at a door where they think there might be baddies inside they will perhaps stop moving in Diamond Formation maneuver and instead the Squad Leader might instruct his squad to stack up and get read to execute a Dynamic Entry maneuver.
Once the room is cleared (with bonuses from the Dynamic Entry maneuver) the squad leader might instruct them to again line up in a Diamond Formation to be better prepped in case of an ambush while moving further into the facility.
If the squad later gain high ground on a group of baddies that haven't noticed them yet the Squad Leader might instruct the squad to again break up the Diamond Formation and instead set up an ambush by instructing squad members into Crossfire positions before all of them open fire at the same time.
2. If I enact Shield Walk, would I need to have someone with sheild to stand in the front? Or someone with shield is good enough?
If one person have a shield then it seem as if that person will gain a DR advantage (even without SUT).
But say he got SUT knowledge then it seem as if he can spend a major action to instruct a squad member (that will spend a major action to listen to the instructions) to move in unison. As long as they are now both moving together in a Shield Walk it seem as if they will now both take advantage of the extra DR from that one single shield (that normally, without this SUT maneuver, would only benefit the guy carrying the shield).
What happen if I got flank?
If enemies are successfully executing a Flank maneuver against the Shield Walking pair then it seem as if all participants in the Flank maneuver receives +2 to their AR (or +3 if the flank was undetected) while the flanked Shield Walking pair would get their DR reduced by 2.
Would I still have the buff from Shield Walk?
As long as you can take advantage of a shield you will typically receive extra DR from the shield. In some situations a GM might rule that you are not getting a DR bonus from having a shield. Flanking might or might not be such a situation (there is no rule stating such, it depend on the situation and GM fiat).
4. If I enact Suppressive Fire, would I need to have a character keep spraying bullets each turn?
The Suppressive Fire maneuver does not provide it's bonus once you decide to no longer provide Suppressive Fire.
If they do need to spraying bullets each turn, would that required major/minor action and ammo to do so?
A regular Major Attack Action with a weapon that is set to Full Auto Firing Mode (which seem to consume 10 rounds).
Or a random BF attack would count?
This SUT maneuver seem to require that participants are wielding weapons set to full auto firing mode (not burst firing mode).
What happen if they run out of bullet or stop firing?
The Suppressive Fire maneuver does not provide it's bonus once you decide to no longer provide Suppressive Fire. This is why Suppressive Fire is typically best provided with belt-fed MGs.
when would they spend the major/minor action? in their own initiative? Or by the time the leader roll combat maneuver? Or else?
It seem as if all members that also wish to participate in the same Suppressive Fire maneuver (if more than one) need to save up a Major Action. The combat maneuver then seem to be "performed on the lowest initiative score of the participants".
But if you have the SUT knowledge skill and just want to provide Suppressive Fire yourself (because you are perhaps also the only guy in the squad with a LMG) then it seem as if you just spend a minor action to activate Suppressive Fire maneuver (because if all team members participating in an SUT maneuver have the Small Unit Tactics knowledge skill, combat maneuvers are treated as minor actions) and then you take the maneuver test. Number of hits on the test seem to indicate number of suppressed targets that will have their DR reduced by half the base DV of your LMG (and AR by half of that). If you get 4 or more hits it also seem as if you gain a situational Edge due to that.
Then you spend regular Full Auto Major Attack Action(s) to actually attack.
teammate without FA weapons, can them be a part?
It seem as if a firearm with full auto firing mode is required if you want to provide Suppressive Fire.
How should I calculate the buff? Who's weapon is "the weapon"?
If you are wielding the LMG and is providing Suppressive Fire then the debuff your suppressed targets receive seem to be calculated from the base DV of your LMG.