The way I understand it, only Gates have Phantoms, so only they have magical power. However, Nitou is different in that because he became a de facto Gate due to Chimera being inside him.
This debate about Chimera has gone on for far too long. This show has had bad pacing. There are several things they could have done to pad out the plot if they only wanted to give us little bits every now and then or loading all the plot into the end of the show, which is where it seems we are going.
There needs to be some in-show explanation of what Chimera is and how that alters Nitou.
Here are 4 options.
1. Chimera is a Phantom that was somehow sealed.
2. Chimera is not a Phantom, he is some other magical creature
3. Chimera is an ancient-Wizard created Phantom, that MUST eat other Phantoms, possibly in the hope to destroy all Ohantoms and then Chimera eats its user and goes dormant to be used in the furture if other Phantoms arise.
4. Chimera is an ancient-Wizard created Phantom-like creature, possibly created from bits of other Phantoms and given a benevolent, anti-Phantom will.
For Nitou being a Gate or not
1. He is a normal human who uses magic through enchanted items, which use Chimera's Mana
2. He has become a Wizard like Haruto by having a Phantom inside of him, and now has his own Mana, which is close to nothing, which is why he depends so heavily on chimera's mana and must constantly recharge it
It's almost a distinction without a difference save the fact that while Haruto's caloric intake (which is increased, like he's pregnant) feeds Dragon, Nitou can not feed Chimera off of his own calories.
To me it seems like Dragon and Haruto share a baby in the womb/mother relationship, while Chimera and Nitou have a parasite/host relationship.