So, I was wondering again (heh, you know me, always asking questions), are all Toa equal in power? Save the Toa Nuva and Tahu.
Depends on what kind of power is in question. In capacity of Elemental Energy, they're all the same. However, in terms of the end result of what they do, more learned Toa will be better at it, and you could call that "powerful" in a sense. See this quote from the Toa page of BS01:
Proficiency with an element is a learned skill, with more experienced Toa able to perform a wide range of tasks.
Some have theorized that as a Toa gains skill, their EE might recharge faster, but this is not confirmed as far as I know.
Also, why were the Toa Mata made into Toa Nuva?
The official answer is essentially that there is a reason contained within destiny, which is apparently not knowable beforehand (same with almost any transformation by EP, other than a few reliable results like Kraata becoming Rahkshi armor).
As for why the EP tubes were there, I believe Greg originally said it was a death trap, and it may have been revealed who set it, but I've recently been unable to find info on this on BS01, not sure if I'm remembering wrong or what. I think (but don't quote me on it) that it was the Order -- a trap for anyone not destined, with the justification that a good guy will be more likely to be destined, etc. (So I guess they were assuming Kraata weren't going to take on the Bahrag. )
As for why that destiny, the easy answer would be that it was seen as needed to succeed at their Toa destiny. Who saw that is the main question, or it could have been merely a contingency, etc.
One more thing, in a Greg F question, it said that now that Tahu had these powers, he would stand a very good chance against a Makuta. But wouldn't he, as a Toa Nuva, already be able to stand near close to a Makuta?
Only slightly. Being a Nuva has these benefits -- slightly better mask power, ability to share it, higher capacity of EE, and greater control over the element. None of those things approaches a Makuta in power level other than slightly. However, multiple Toa Nuva working together would be much more effective than the same number of normal Toa, due to the ability to share all their mask powers with each other at once. But as a Nuva alone, no.
One clarification: Makuta aren't as powerful as they might seem, though, since they can only use one Rahkshi power at a time.
But they seem to have a virtually unlimited energy source for this, and have many other advantages as antidermis, plus the protosteel armor, etc. They can use any number of powers to fit a wider variety of situations, and that's the biggest factor in their favor. They're not as susceptible to the kind of situational pressures that I've described before as other beings. Put most beings of power in the wrong situation (control over stone on boats on the ocean for example, against a Ga-Toa), and they'll simply be at the disadvantage. But even with any Toa in their most opportune situation, a Makuta will have multiple options to match that situation (including Adaptation itself).
Still, it's possible a Makuta might find themselves in some kind of situation where a single Toa like Tahu might have enough of... not really an advantage but a level playing field to have a fighting chance, even in a no-holds-barred situation. Perhaps a Toa of Fire would be one of the best -- they could conceivable melt a hole in the armor and then burn the antidermis (one of just a few ways to kill a Makuta) -- but only enough to severely weaken (something like this may have happened to Teridax in the past, when he was weakened down to Turaga-Dume-size). Obviously, the Makuta feared Toa of Iron for a similar reason, which is why they were slaughtering them.