2004 monte carlo ss running hot on start up. Replaced thermostat and radiator cap.

It really runs hot within a minute of starting ?
An engine that runs hot in a few minutes is often having problems with the water pump or a blockage in the cooling system. The thermostat does not open until 195 degrees which takes 5 or 10 miles of driving.
During that time, the water pump should be circulating coolant in the block and the heater hoses.
A leaking head gasket should be putting extra pressure into the cooling system, and-or dumping coolant into the cylinders which would come out the exhaust pipe. But it should still take a few minutes for the engine to reach the boiling point of 240 degrees.

it sound like you could have two problems one is you might have a bad head gasket two u could have a bunch of air in you radiator so i would start the with the radiator cap off and see what happens if its air this should take care of the problem

Check for air trapped in system,and not to be a smart a-- but could thermostat be in backwards?

Was the water pump leaking? did you overheat the engine? you may have head gasket issues. If you can bleed th eair out and it works ok a few days, then it has air in it again, most likely you have a bad head gasket.

Check the thermostat, if it isn't opening when engine warms up, the coolant in the engine will overheat and make pressure in the cooling system but the fluid in the radiator will still be cold.

it may be time to put a new thermostat in,that will bring the engine up to operating temp quicker,and your vehicle computer and sensors are designed to be at 195degrees

Better make sure the fan works. There is a switch on the radiator that turns the fan on when it gets to a certain temperature. It could be the fan or the switch. Your thermostat is not your cap, it's in a different location. (Usually at the engine end of the upper radiator hose) Get the leak check! A good garage can pressure test the system to see what leaks you might have.

change the radiator cap. its a 7 dollar part that generally cures the problem. (the silver cap on the top left side of the radiator).

Thermostat is probably stuck closed... Run the car with the cap of the bottle or radiator and see if it starts to circulate when it gets hot... Be careful though because it can over heat and have no coolant flowing.. And also watch for spewing fluid if it get hot at the place you took the cap off.... The low coolant could also mean the radiator is not full but the bottle is... Check for a restriction from the bottle to the radiator.. If you see a sensor on the bottle it could be bad... But That won't cause it to over heat. It'll just say low coolant because the switch is stuck.
Also does the fan come on while it's getting hot?
If not that could be the reason for the overheating.. And the coolant level sensor could be bad telling you it's low
Cap off look for circulation of antifreeze.
At the same time before it over heats make sure fan is coming on. No fan on? Fan fuse or relay could be bad... Also maybe a wire.
No circulation? Thermostate stuck or restriction in coolant hose...Line... Radiator..... And worse case scenario the water pump might be bad.
The low level I wouldn't worry about. Find out why it's over heating first.