First note that officially 灯 has only ひ as kun-reading (とう as on-reading). Officially means other readings are not listed in 常用漢字.
- ともし is equal to ともしび. I don't think this reading is used today (as the reading of single 灯).
- ともしび means a fire for light, typically the fire of a candle. An idiom 風前の灯火, literally a (small) fire in front of wind, means something is in great danger.
- あかり is a general term for light you need in darkness. As you are aware, in most cases you can stick to this for light.
- ひ is just 火, fire. If you need something to light cigarettes, you can ask your friend 火ある？.
Regarding あかり and ひ, most probably it will be understood as 明かり and 火（or 日, which has a different meaning). If 灯 is used for those readings in a novel, say, then it suggests the light of torches, street lights, or something similar.
On how to read 灯
If 灯 is used alone, then on-reading とう is unlikely. Between あかり and ひ, り tends to be added for 灯り for disambiguation. And ともしび tends to be written as 灯火, so the chances are that a single 灯 is ひ, but there remains ambiguity without a ruby. In general there are fluctuations in okuri-kana (送り仮名), and same words can be written differently according to the author's styles or tastes (e.g. 終わり / 終り).