Is growing natural weapons a minor ability as per the level 2 MuCo guideline? Also how limited would this be? Growing large claws (as per RoP:F for example) seems to be the most desirable option.
MuCo 5: Growth of Ferocious Claws R: Touch, D: Diam, T: Part Req: Animal
The target's hands grow into large claws (with combat stats as per RoP:F, p.48) that can be used with the Brawl ability. For the duration of the spell no fine manipulation is possible.
(Base 2, +1 Touch, +1 Diam, +1 Part, requisite free)
... actually Part may not be necessary as per Eyes of the Cat (ArM5, p.131), but I prefer it this way.
Depends on how "minor" the weapons are i would say.
I´d say that growing claws are probably a "minor ability" without any problem.
If, for example you wanted to give yourself something much more serious or complex like say, the ability of a Bombardier beetle, upscaled, then I would certainly demand a higher base level, probably 2 higher.
I can't see anything wrong with this Toa. Given that the impact of such an ability is fairly balanced when compared to a Magus carrying a standard weapon, and also physically changes the target which marks them as clearly unnatural (for mundane reaction purposes), it seems that level 5 effect is fair and reasonable. Claws are not an unbalancing power at all.
If you need a way to moderate the impact of power, add some fluff about the use of tools and such while the effect is present.
In terms of how far a "minor power" can be stretched, it should be a single part or aspect of a creature, which can be used in an obvious way. Claws of lion yup, eyes of cat yup, smell of dog yup. Even spitting of a snake seems ok when the charatcer also accepts the physical changes which would accompany it.