Electrical – connect 12 gauge wire to 18 gauge wire for an outdoor light post and plug

If the 2 prong receptacles are existing receptacles, there shouldn't be a problem. As long as you realize the 2 prong receptacles will not have a ground, and you don't replace the 2 prong receptacles with 3 prong receptacles.

In your situation where you have one neutral shared between two circuits, you can't use the "load" side of the GFCI receptacle.

That's because stuff downstream on the other circuit will feed return current on the neutral, so the GFCI will see an imbalance and trip. To put it another way, if you have 5A of current on the red wire and 2A on the black wire, you'll have 3A on the neutral wire (the red & black cancel, with the remainder on the neutral). Since the current on the black & neutral aren't in balance, the GFCI trips.

There are two options in this situation:

1. Install a 240V double-pole GFCI in the breaker panel. This type accounts for varying amperages in the two "hot" wires. This is also the expensive solution, and it's not ideal because if you trip one circuit the other one trips, too.
2. Replace all of the receptacles on the circuit with GFCI, using only the "line" side of the GFCI. Use wire nuts & pigtail jumpers to join the wires coming in & going out of the box. This option is more work, but may be cheaper. It's also more convenient because if you trip the GFCI on one receptacle, only that receptacle should be affected.

The refrigerator ideally should NOT be on a GFCI, because you don't want it to trip and ruin all your food. However, if the fridge is plugged into a GFCI that's within 6 feet of the sink, it still must be GFCI protected because of the proximity to the sink (though it may not be a problem if it's behind the fridge or is a single rather than duplex receptacle).

Here's a good primer on how GFCI works.