There have been many questions about the Assad regime using chemical weapons against the rebels opposing him. A common rebuttal from the Syrian government and its allies is that the rebels are the ones using chemical weapons and they had a stockpile of weapons that was hit by a more conventional weapon causing a chain reaction.
Is there any evidence that the rebels in Syria have ever used chemical weapons against the Assad government or against ISIS?
Yes.
One of the most reported chemical attacks by the Syrian Rebels is their chemical attack on the Kurdish controlled Sheikh Maquood neighborhood of Aleppo during the Siege of Eastern Aleppo on April 7th, 2016. According to this source, the chemical used was yellow phosphorus. Although no international organizations were able to confirm the attack occurred, the rebel group Jaish al-Islam admitted that one of their commanders used "weapons that he was not authorized to use." This statement by Jaish al-Islam is dated on the same date that the chemical attack supposedly had happened: 
There are also other claims with a good list of chemical weapons claims provided on wikipedia
