There are quite a few NPC that can play more than a 1 scene role- Aneevia, Arushavail, Tirabeth as main examples. I understand how they can be helpful if the party is missing a "role". But if the "core 4" are filled by PCs do the NPCs take away their turn to shine?
No
In part 1 Aneevia and Arushavail are useful but their injuries mean they cannot take the lead. In book2 The PC's mythic power makes the others useful adjuncts at best. My players did not risk them in combat much in book 2 and by book 3 they are hopelessly outclassed
The only NPC that has a chance of "outshining" the PCs is Arueshalae, but the restrictions on her actions as she seeks redemption tend to limit her impact. She is definitely more powerful than a PC when they meet her and possibly gain her help toward the end of the third adventure; after that, she is basically "off-screen" for the most part until the PCs hit 15th level/7th tier.
She is still probably a better archer than most PCs, unless that is their dedicated focus.
The other NPCs shouldn't gain mythic tiers. At most, if they will be accompanying the PCs on a regular basis, then give them the Mythic Companion feat; they are meant to be allies and (possibly) cohorts, not GMPCs.