I believe that al Qaeda is constantly looking for ways to become established in different countries. By alinging themselves with the rebel forces would ensure that they win that they would have a presence and some influence in Syria. It would make sense that al Qaeda would have a presence in this conflict.
I believe Syrian rebel forces include al Qaeda members. This is because of the "the enemy of my enemy is also my friend" way of thinking. It would not be far-fetched to believe that radical Muslims with different direct aims would join the same groups to gain more power and get more unity.
Contrary to what politicians sometimes try to make us believe here in the United States, al Qaeda is not a monolithic organization that resides in one particular country or in one particular area of one country. It is a terrorist movement that can be found wherever there is unrest and most assuredly is in the Syrian rebel forces.
In Syria, it is impossible to escape the influence of al Qaeda. The group and its affiliates are very involved players in the entire Middle East region. The Syrian conflict is one of the largest conflicts in the region, so they must be involved somehow. Any group that seeks power will go to where there is a possibility to gain control.
The Syrian rebel forces definitely have some al Qaeda members. Of course they do. The intelligence supports this idea, and al Qaeda is looking to rebuild in any way they can after the death of many of their leaders, and especially Osama Bin Laden. Don't be surprised to find them in many rebel groups.