it should be ok to houserule. The problem is that it has to be balance well. The torch you have to discard The sword in the expansion is a yellow level only
ignite is a blue level and a no discard item, which could make it OP. Also (sorry no at home, so cant check) but is it that the starting spell? which would make it really easy right out the bat to kill abomination. Which in my opinion I`m REALLY greatfull that they remove damage 3 weapons and made the abomination scary again, by the end of s3 it was so easy to get a weapon to kill them that they became a regular zombie. Hence imo using ignite (specially if the starting spell) to light up dragon bile, i would put it that only happen in die roll 6; MAYBE 5+. And maybe even in a once per game, with a easy side the card or put a token on it to keep track of it.
We played Quest 1 yesterday and ran into this. We had the dragon bile but not a torch. The wizard had the fireball spell and we felt like that should be able to ignite the dragon bile. It's a fireball spell, fire is in the name. Otherwise the game would have just devolved into searching for a torch round after round.
So we house ruled it that a fireball could ignite the dragon bile.