Mixing wire gauge questions
Think of wires as "bad" (or weak) resistors. For every foot of wire, you incur some amount of resistance, which imparts a voltage drop (read: power loss) to the system.

Extension cord wire gauge should be chosen based on the amount of current that you intend to draw, and the total distance needed. There are charts which provide this information, such as this one at the Engineering Toolbox:



Such charts are derived by using the wire resistance, distance, insulation type, allowed increase in temperature, etc.

A thicker wire (lower gauge) has less resistance, and thus less power loss. If you first determine that your application can use a particular gauge for the entire length needed, you are safe to substitute a heavier gauge wire entirely or in part.