probably a dumb question but can you combine ddr2 with ddr3 imac memory?

DDR2 and DDR3 ram are physically different. A slot for one cannot accommodate a module of the other type physically.

Notice how the pins on the left side are different, and as such the notch is in a different location on the module.

http://www.crucial.com/usa/en/support-memory-motherboard-compatibility

Note, however, that you may be able to use a faster rated DDR3 in a computer that already uses DDR3, but not slower RAM than what the computer requires.

No. You cannot do that, it has to be the same.

No. Only one of those types(and for some models, neither) will fit into any given Mac's RAM slots.

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