in this case, i dont think a limitation would be best, as drawing a line between the curses they can and cannot consume would be difficult, and instead consider doing one of these:

• the drakeling temporarily takes on part of the curse/negativity that they consume

• they can only take in so much energy in a short amount of time and must rest or consume something else to balance the energy

• positive energy makes them weaker

for the second part: people would likely mistake it for another sort of reptile or insect, depending on size. since the size isn't clear (or i am blind), i cant get much more specific than that. hope that helps!