Phantom was actually mutated by an “Energized Protodermis variant,” so I doubt he still qualifies as a Matoran. (Still bugs me we don’t know what he looks like; that might help answer the question more conclusively.) I said he might be able to for that very reason. No idea how extensive his change was. For example, the Toa Mata became Toa Nuva, without changing much of their physiology. It was stated that if Radiak in his mutated form were to become a Toa, he would retain his wings and four-legged shape, so it depends on the extent to which Phantom was changed by the EP. If all it did was make him bigger and stronger, with some minor powers, while still retaining his inherent nature as a Matoran, he should still be able to become a Toa if destined to, but he wouldn't look the same as any other Toa, like Radiak. Well then Firedracax should be able to become a toa as well, as the mutagen that changed him was also Energized Protodermis. However on reflection, considering both where destined to change, I think that there is very little chance of them becoming Toa, although I don't see why not if that was also their destiny. Also I love the idea of a a four legged Toa Radiak for some reason.