Okay: There is a weep hole under the water pump. If the seal or bearing in the pump is bad, it will leak from there. If you are removing the pump anyway, I would replace it. As far as the thermostat, that is an easy job, and the belts will already be off.
Personally, I recommend using a GO-5 universal coolant. That way, you can add other types to it if needed. The original DEXCOOL (not dextron. Dextron is trans fluid, please don't use that lol), was not one of GM's best ideas. Corrosion was a problem when using it. Plus, if you don't flush the block, some still could be in the engine block and cause issues if you use a different type of coolant (other than universal). I do recommend flushing the engine block and heater core while you have the thermostat out and water pump off. Just get a engine flush product as well as radiator flush and do both.