If I may throw a GIANT wrench in the works, I play an Untramarine Tactical Marine that is focused on squad mode abilities. Let me lay out some special things I have collected to let me ignore most all of the rules when it comes to squad modes. Sometimes I find that learning a concept is easiest when I know what the exceptions are...
Tactical Expertise from being a Tactical Marine: Make a Command test to let one of the squad modes you have access to affect your whole team that is in range (also based on rank)
Defence Genius from Rites of Battle Deeds: GIves me access to Defensive Squad Mode abilities from any chapter that is in my kill team. My team is the following currently: Myself Ultramarine Tactical, Dark Angels Apothicary, Space Wolf Tactical Marine, Storm Warden Techmarine, Blood Angel Assault Marine. This means I can use the Defensive Squad Modes from Ultramarines, Dark Angels, Space Wolves, Storm Wardens and Blood Angels.
Strike Team Specialist from Rites of Battle Deeds: GIves me access to Offensive Squad Mode abilities from any chapter that is in my kill team. My team is the following currently: Myself Ultramarine Tactical, Dark Angels Apothicary, Space Wolf Tactical Marine, Storm Warden Techmarine, Blood Angel Assault Marine. This means I can use the Offensive Squad Modes from Ultramarines, Dark Angels, Space Wolves, Storm Wardens and Blood Angels.
In closing, these three abilities put together mean that I can use "Lightning Strike" from the Storm Wardens (access through Strike Team Specialist and the Storm Warden team member) and have it affect everyone (Tactical Expertise) with a Command check. I have Fellowship of 65 (rolled a 50, +5 from Ultramarine +10 from advances) and the Command +20 talent (Ultramarines) and a few other deeds/items that put my command check at a clean 100 so I can't fail (except snake eyes). I may not bring a lot of pain myself but i can generate 16 cohesion a session which is lots of "Lightning Strike" and other fun abilities for the whole group to take advantage of.
We came upon another problem with my massive amounts of Cohesion, any single player may only benifit from one Sustained Squad Mode at once. This means that we had "flashcards" to tell which they were under at any given moment. When using a Sustained Squad Mode you can still be affected by Non-Sustained Squad Mode abilities like Lightning Strike so no worries there. Just a helpful hint if you go down the Cohesion rabbit hole a wasy further, use Sustained Squad Mode abilities at the very begining of your session because they last all session. Even if no one is "using" it at the moment you can still change back to it with no additional cohesion cost.
In the end, Cohesion/Squad Mode is a fantastic force multiplier but requires a little bit of accounting on the team leader's side to make sure you do it right. The only downside is all of the fantastic and fluffy Solo Mode abilities that are never used by my group because they want my +6 to actions and 1 reroll (that is one reroll for the whole group BTW not per person).