[ek_hornbeck]

but when al Qaeda operates in an area for years on end, and then ISIS pops up in the same exact location, it’s hard for me to think they’re not jumping between squads

This is certainly true, but both Al Quaeda and ISIS are Sunni organizations. What you don't get is fighters moving from Hizbollah (Iran-sponsored Shi'ites) to ISIS (Sunni) or vice-versa, as this would basically require a "denominational" conversion. Think of Catholic vs. Protestant during the 30 years war.

[DoughtyOne]

As I understand it, they are both Sunni, but ISIS was more of a Wahhabi sect. It doesn’t see other sects even in the Sunni grouping to be it’s equal.

Thus I believe the commingle argument is valid.