ISIS and Hamas are kind of enemies since ISIS challenges the rule of Hamas over Gaza.
They believe that Hamas are not radical enough.
However looking just at big organizations for Arabs and Palestinians in particular is not the right way to gauge stuff. While IS Gaza could never dream of wining against Hamas they don't need to.
It's all about which tribe supports who, and some of the strongest and most vicious tribes in Gaza have switched to ISIS side. They still remain vastly inferior in strength to Hamas and Islamic Jihad or even most likely to smaller organizations, but it's interesting to see how other tribes would swing.
The tribe that joined with ISIS is the one whose men that originally kidnapped Gilad Shalit (they POW who was a matter of a lot of fuss a while back and who we released 1k prisoners for giving Hamas a lot of street cred).
Hamas' position is precarious now as they have become quite corrupted on one hand but struggle to appease the population after last summers' operations.
For example despite hundreds of tons of building materials flowing into Gaza from Israel since last summer not a single house was rebuilt, while the Hamas elite a lavish life.
Their funding is running short for all their needs since Iran has limited their funds after they expressed support to the FSA at one time, and Qatar funds dried up somewhat as per pressure from KSA. UNWRA also has shut down a lot of relief operations there due to lack of funding.
So while they have an overwhelming military strength they are still on rocky ground.