I wasn't going to bother posting here, to let somebody else bring this up, but surprisingly I haven't seen it mentioned -- IMO, the answer to this is probably very simple. Velika likely chose a destined Matoran as the body to inhabit simply because it's good strategy to always have the option of transforming into a Toa if he finds himself in trouble.
This may imply he had access to a Toa Stone or some equivalent, maybe one laying around in his backpack, perhaps an object that doesn't look like it would be a Toa Stone, or maybe even a part of his armor. But it might have been lost in Karzahni, and he might have decided that wasn't a safe place to become a Toa (Greg has said that he doesn't believe that Velika intentionally got damaged and sent there).
And more interesting -- maybe he plans to transform into a Toa when (or before) he takes control of SM? (Or maaaaybe he already has? Not sure if we know he's still a Matoran, offhand...)
While I'm at it, might as well clarify two other things that were brought up here:
because Velika was the reason for the Matoran Universe's inhabitants achieving sentience.
He had no hand in the MU inhabitants developing such a high level of sentience.
Both of these statements make the mistake of "unwarranted certainty". What we know is that Greg expressed great interest in a theory somebody asked him about in the LEGO.com topic that Velika caused it (BTW, the term is sapience, not sentience, although a "high level of sentience" is a fair definition of sapience I suppose). He didn't say one way or the other, so at this point we should consider it a possibility, but definitely not confirmed.
(I suspect it's more likely Greg liked the idea of making it look like Velika caused it in a possible story that he probably won't ever have time to write. Well, continuing the serials. Anywho. Point being, I doubt he'd end up having Velika cause it because Velika was in there to observe and Greg also stated he didn't want to interfere with Mata Nui's mission, and activating full sapience might be too unpredictable for that. Plus it enabled the Makuta rebellion apparently, which Greg said Velika didn't like.)
There's no reason that there can't be multiple masks of light, as it is just a "Great Kanohi", not (as I would've thought a few months ago) a Legendary Kanohi.
BUT ALSO: Tanma doesn't need a MoL to be a Toa of Light; he's already an Av-Matoran.
Just to make sure everybody reading here knows this, the Avohkii transformed Takua because it had been charged up with Toa Energy; it's confirmed that Toa can charge up any object with TE (usually they use stones; making "Toa Stones", but they could use any object), from a limited, non-recharging amount they have. It had nothing to do with being a Mask of Light in any physics sense. Light was chosen as the power for the mask because that TE charge was intended for the destined Toa of Light, so it was symbolic (and it would be practical as a backup Light power if the Toa of Light's Elemental Energy ever ran low, plus the mental powers gave extra abilities).
In other words, there's absolutely no reason to expect any association between a Toa Tanma and another Mask of Light.
Also juuuuust to be clear, because of the phrasing of the last clause in this quote, the Mask of Light doesn't change people into Av-element beings either. All it does is act like any elemental mask when worn by a Toa, and previously was charged up like a Toa Stone (that TE is now depleted). (Plus, Artakha added a knowledge-downloading feature, but that wouldn't be a normal part of a Mask of Light either.) Takua became a Toa of Light only because he was an Av-Matoran, destined to be a Toa, and he got access to a source of Toa Energy (plus we think you have to actually make a choice to accept the responsibility, explaining why he didn't previously become a Toa when the TE from the six Toa Stones he collected hit him). We proooobably all knew that but hey, just in case.