I have contemplated, and utilized, candles for CO2 supplementation in prior grows. I used small candles in containers, and they increased the ppm in my tent significantly. I used them in the winter so that the increased temperatures were beneficial. I cannot say that I was worry-free, but they performed well.
I didn't have a problem with soot, but I didn't use them for long. The CO was not a concern, as plants can metabolize CO and the minuscule levels were negligible. I did find that the constant replacement of candles was a chore. I tried a few other methods of CO2 generation.
I have used yeast and carbohydrates, but the odor and mess were a strong deterrent. I used a Coleman lantern, which was much better, but the heat was an issue. I considered using exhaust fumes from my pellet stove, but the logistics were prohibitive. I finally settled with bottled gas.
The CO2 cylinders work great with a timer if your environment has a large gas exchange. My exhaust fans run continuously to remove heat and water. Bringing the CO2 ppm up is a challenge. I use an infinite cycle timer set for 5 sec bursts every 5 min. This elevates the ppm up to 500 at the foliage before it is sucked out. It's not optimal, but it's safe, economical, minimal maintenance, and worry-free.
I can see using candles for interim CO2 elevation, if watched carefully. I would recommend investing in a tank and regulator; the benefits outweigh the detriments. But I'll tell ya that the stench from the yeast metabolism of carbohydrates, either aerobic or anaerobic, permeates clothing, your neighborhood, and even your burps will smell like bread.
( - I warned you about the yeast...just say "NO".)
Nah man, you added good stuff