I've seen te text statteing that marines cannot benefit from more than one squad mode ability per round or at once. After looking at the description for solo mode abilities I only see that they can be "used freely".

I take this to mean that a character can simultaineously be benefitting from multiple solo mode abiliteis at the same time. For instance; A marine in melee with a dangerous foe might use Righteous zeal and Feat of strength and Emperor's grace. Also, since activating these specific abiities are free actions, our hypotheitcal marine (who by his solo modes we can deduce is a level seven or higher black templar) can kick all three of them off in the same round unless the GM thinks that this exceeds a common sense reasonable limit on the number fo free actions taken in a round.

Is this right? Can my starting marine activte both boost of speed and feat of stength in the same turn to rush that extra few meters to his target and whomp it extra hard?

A character cannot benefit or sustain more than a single sustained mode power at once, unless the power is passive.

A kill-team may activate any number of non-sustained powers at once, BUT a character can only benefit from a single Out of Activation action a turn.

This means if X character is sustaining Power (a), and Y character is sustaining Power (b); X character can not benefit from Power (b) unless he stops sustaining (allowing to expire) Power (a), and vice versa. Space Wolf Player Z has his passive smell power, which is always on, but is not sustaining any active powers and may therefore choose to benefit from either Power (a) or Power (b), but not both.

In Turn 156, Marine X decides to declare Furious charge, Marine Y decides to declare Regroup, and Space Wolf Player Z activates Bolter Assault. All three powers activate, as a team can activate as many non-sustained powers as they want at once, BUT each character may only use one of the three since they each allow an out of activation action, and characters can only use one Power-based out of activation action each turn.

If Marine X activated Furious charge on turn 157, Marine Y activated Regroup on turn 158, and Wolf-boy Z activates Bolter Assault on turn 159; then all the players can benefit from all the powers as they are all on three separate turns.

Your reply specifically concerns squad mode abilities. From my reading the same constraints do not apply to solo mode abilities. Do you have any specific spots in the rules where it says this? Do you have a page reference?

RoB p. 225: Passive Solo modes on all the time; Solo Mode Abilities may be activated freely when in Solo Mode.

RoB p.226: Anyone can activate Squad modes.

From Tim (FFG Rules Clarifications)
"...only the Battle-Brother who originally activated the Squad Mode Ability can sustain it. Other Battle-Brothers may benefit from it, but they are not in control of the Ability, and thus, cannot sustain it. This goes for Chapter-specific Abilities as well as Codex Abilities."

"When a Battle-Brother leaves Squad Mode, for any reason, while sustaining a Squad Mode Ability, the Ability is no longer sustained and its effects end as if he had chosen to stop sustaining it at that moment."

"...in the event that you leave squad mode for any reason, you do, in fact, have to pay the Cohesion cost again when you choose to reactivate the Squad Mode Ability.

One clarification should be made about the current errata for "Sustaining Squad Mode Abilities (page 220)" on page 5 of the Errata v1.1 document. The sentence "The Battle-Brother may choose to stop sustaining and [sic] ability (possibly to activate a new Squad mode Ability) as a Free Action at the start of any of his Turns" should change to "The Battle-Brother may choose to stop sustaining an ability (possibly to activate a new Squad Mode ability or benefit from another active Squad Mode ability) as a Free Action at the start of any of his Turns and must pay the Cohesion cost of the sustained ability if he wants to reactivate it at later [sic] time. " The following sentence should be added to the end of the description: "A Battle-Brother may only sustain or benefit from one Squad Mode ability at a time."

Keep an eye out for a future Errata and FAQ for the above clarification." >.>

The single Out of Activation Ability Action a turn (SOAAAT) clarification come from a response from FFG I do not have personally.

Generally, the rule allowing only a SOAAAT also applies to Solo Mode abilities, but otherwise there is no limit to the number of Solo Mode abilities a character may use at one time (though there are limitations on how many times a battle/day these abilities may be used).