I'd say there are two reasons that indicate that not all Toa of a team have to become Turaga at the same point:
a) Toa have to give up their Toa power and have to have fulfilled their destiny to transform into Turaga. So after fulfilling their destiny, if they still have their Toa power, they can decide not to become Turaga yet, indicating that it's up to the individual Toa to decide when they will transform.
A Toa will turn into a Turaga if they have completed their destiny and sacrifice their Toa Power. If a Toa has expended all their Toa Power before they fulfill their destiny, then they will become a Turaga as soon as their destiny is fulfilled.
From Reign of Shadows:
The destiny of the Toa Nuva had been achieved, so he (Kopaka) always had the option of giving up his Toa power and becoming a Turaga. But he had no real wish to wind up running a village or outpost somewhere.
b) It's indicated that at least one Toa from Jovan's team stayed a Toa after they fulfilled their destiny since he/she was killed as a Toa.
Several of the survivors became Turaga.
(Members:) A Toa who wore a Kanohi Olmak and was killed by the Brotherhood of Makuta