I know the coolant system is a high pressure (closed circuit or something) system. With that if you just wanted to top it off...is it ok to just unscrew the overflow tank and fill from there? Doesn't that introduce bubbles into the system?
No it won't. There is always air in that tank but it should be above the level of the coolant. The pickup on that tank that sends coolant into the coolant system is on the bottom of the tank and so long as the level of the tank is maintained where it should be no air should be able to get to that pickup. There is an overflow tube that goes from that tank to the outside even if the system over pressurizes.