The Ultramarine defensive stance 'Rally Cry' allows to restore fel bonus of cohesion points. It can be used once per session. But what happens if

- the whole Kill Team consists of Ultramarines? Can each of them use this stance?

- The leader of KT is a Tactical Ultramarine, he passes his Command check and thus he can share benefits of his ability with the whole team? Does it mean that everybody can now use Rally Cry once per session? Or am I mixing stances with abilities?

Every Ultramarine can use it once per session I believe

and you aren't confusing abilities. the tac marines ability allows everyone to benefit from a squad mode, It does not allow anyone to activate any squad mode. usually a squad mode ability will be like 'everyone gets an extra shot' or whatever but in this case because the squad mode ability only affects the cohesion pool I would say tactical expertise cannot affect it.

The only other option I can see is to have two pools of cohesion, one for the people who are affected and one for the people who aren't. but given that cohesion represents how in sync the marines are this seems really stupid

I'd concur.

Each squad member gets to use the ability once per session. I guess that this means that Codex Chapters have REALLY good teamwork due to the sheer amount of Cohesion that they can spend on Codex patterns. Which...makes sense.

And in the second case: No. I'd not allow it to work like that. Only the Tac could use rally cry, but all others could get the bonus from the Sustained Chapter ability.

The only thing is that with the squad of U Marines is that Rally Cry is only usable once per gaming session... Wouldn't it be a little broken if they would just use it at different times throughout the session? Like say there is 6 different battles planned by the GM and they each used the ability in a different battle. I think it cheapens it that way. It's supposed to be something that brings every one together by boosting cohesion. Any more than 2 times a session would belittle the rally cry, to me, to the effect of: Didn't we just do this...? Another example, in real-life, would be like a pep-talk before a game. Having more that one pep-talk would be pointless... This is prolly just my ramblings but does anyone see where I'm coming from?

Honestly, if the entire squad was composed of Ultramarines (or any single Chapter), I wouldn't even use the Cohesion rules. The reason for Cohesion is to reflect marines of different Chapters learning to cooperate and fight together; brethren of the same Chapter don't really need to learn to do that. At the very least, I would add a substantial Cohesion bonus to the total starting Cohesion (one point per Marine) if the entire squad/kill-team was composed of brethren from the same Chapter.

A tac can let others (of another chapter) benefit from the chapter squad ability he uses. Rally Cry already gives the KT cohesion (and no other effect) so it's not affected by the tac's ability, like say Lead By Example would be (which gives bonuses to those affected, and is sustainable).