Can you use a 7.5 v 300 mA adapter if device calls for 7.5 v 700 mA?

No because the adaptor must supply the required current for the device (or more). A device that needs 2 amps will not work if the adaptor can only supply 200 mA (0.2 amp).

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The adapter's output is 200 mA, the load demand is 2 amps which is the same as 2000 mA.