Mab said Thomas being in (true) love made him "mortal enough" to work as a Winter Knight, so I don't think species is that much of a restriction.
I don't think he's the personality type for it, though. Look at the number of times he's suggested Harry go take a vacation away from Chicago rather than confront some dangerous situation. He's more than willing to face danger to protect the people he cares about, but he's not as inclined to get involved when it's about strangers.
I draw a distinction between "mortal" and "human." Thomas isn't a human, but he's very, very similar—human-adjacent, if you will.
Regardless, I agree that Thomas's personality doesn't lend itself well to him being a Knight anyway. Aside from his semi-selfish nature, he's way too susceptible to temptation, including wrath. We've seen how well that works out with both Murphy and Harry earlier in the series. In fact, I can pretty much see Thomas trying to use the Sword for treachery and breaking it, precisely how Harry did in Grave Peril.
Maybe he could use it temporarily, during the BAT—I could see something like Murphy's Changes Chichen Itza Charge, but not on a permanent basis.
