Is it OK to mix or combine different types of attic intake vents?

Yes, if their location will experience similar wind and pressure conditions low on the roof/in the undereave or overhang. They will work together to be intake vents. For example, you could install The Edge Vent (installed on the roof at the roof's edge or 2-3 feet up the roof in exposed rafter tail situations) in conjunction with rectangular undereave vents (installed in the overhang of the house).