SpoilerA given person's aptitude in magick is determined by their "aetheric potential"/"aetheric surplus". This is the amount of aether about their person which is "spare".

Every living thing has a certain amount of aether which makes up what I guess we'd call a soul. This is necessary to keep them alive, and cannot be used to, for example, cast a fireball; because to do so would strip the person doing the casting of something that's keeping them alive, and promptly kill them. It'd be like ripping your heart out and throwing it at someone as some kind of offensive missile.

Some living things have aether about their person which is "theirs", but which isn't strictly necessary to keep them, you know, alive. This is their aetheric surplus, and this is what is used to cast spells with.

Others don't have an aetheric surplus at all [THM quests cover one such individual]. A person's aetheric surplus can also be temporarily exhausted [CNJ quests show this happening], and trying to continue spellcasting past this barrier will cause death [a fairly major plot point in the CNJ quests].

Now... I've only done one part of the MNK quest chain, so I don't know much about chakra yet. But any form of aether manipulation is likely to require at least a small aetheric surplus on the part of the user.

My instinct is to say that most people who use chakra techniques would also be able to learn to cast spells, and if you wanted your character to be one of them then it wouldn't raise any eyebrows. The only thing is a matter of how long it takes to learn a new discipline, and the fact they'd have to carry a spell focus (like a sceptre) around with them to do it.

SpoilerA given physical manifestation of aether comes about from a given recipe - e.g. using the Fire spell as an example for this post, it comes about from the following recipe:

And that's the recipe for safely doing that thing [producing a fireball and then throwing it].

I think if you tried to use chakra techniques to cast a fireball, it'd end up looking like standard magicks anyway by the time you managed to successfully and safely create a fireball outside of yourself and then throw it at someone.

Because chakra techniques are for chakra, and fireball techniques are for fireballs.

Either that or you'd end up blowing yourself up if you tried, because you fire-aspected the aether before it left your body?? Am I understanding any of this right???

SpoilerI think that because chakra are mostly about enhancing your own strength, there's little to no transmutation required. That, or it's unaspected magic (like, but not, Ruin). Which means you don't need to spend time filling a spell-focus with aether, or transmuting that aether into something like fire. You're just shunting aether around your own body to enhance it, or expelling it as-is with intent to harm. Right?

Either way, with no spell focus and no cast time, the effect you'd produce in trying to (again using as an example) make a fireball projectile would be very weak, if noticeable at all; since, with no vessel to hold the aether as you siphon it off, you'd be having to transmute it in real-time as it left your hand.

Like...
Casting a fireball with a spell focus and cast time =

1. filling a bucket of water [siphoning aether into the spell focus],
   
2. heating it up until it's boiling [transmuting it into fire-aspected aether],
   
3. and then throwing it over their head [throwing your projectile fireball].
   

Casting a fireball without either =

1. flicking lukewarm water in their general direction.
   

The same metaphor applies for healing energies (unless your character has a rare gift in that specific department - again see CNJ storyline) and earth/water/air/ice/lightning projectiles.

And that's assuming it's even safe to cast such things without a spell-focus. Like I said before, I think it's likely that there's always a risk with trying to transmute aether that isn't somehow tangibly separated from you (for example, by placing it into a spell focus). Like, "you might accidentally transmute some of your own soul and blow yourself up" dangerous.

...in fact, I'm pretty sure that's the mechanism by which the posthumous character in the CNJ storyline - who had aforementioned healing gift - died. Albiet not by blowing herself up. Probably.

And note I said CNJ/THM and not WHM/BLM. White Magic and Black Magic are both highly restricted, highly illegal forms of magic that require the prospective user to go through certain channels to obtain the knowledge/soul-crystals that make them possible/safe.

Using black magic without a soul-crystal means you burn yourself alive from the inside. (Possibly because you're transmuting aether before it leaves you? I didn't get that far in the BLM chain yet.)

Using white magic without the permission and tutelage of a Padjal means you're making yourself a huge target for all sorts of nasty forest business.