I never really saw water bouncing in a gauge no matter speed, engine roughness or otherwise.
Having a half glass of water was never considered to be more than a basic comfort area when on any sort working locomotive, half glass was fine when setting your engine up for departure from a yard, especially garratts ex BMD yard with fulll loads.
It was always taught to maintain at least a 3/4 level in the glass, on flat & light steaming which meant you had safey margins for any harder grade ahead & to keep the fire set & steam pressure at max, without unnecessary blowing off. Whe going up a grade with regulator open the gauges showed more than actual water in the boiler & the drivers side gauge had a different measure to it, I forget the actual term used, but actually meant it was more a stable read of the water. If you had a 3/4 glass & shut off. it would drop to around 2/3rds.
When in working, the blower was always kept on even when steaming hard, as it helped with burning the coal more in an even manner then having it turned off, & less likely to clinker the fire, only time it was ever turned right off was when stabled, as it helped ensure minimal water use & loss, in these locations including depots, 3/4 was the minmum to be maintained in the boiler.
Different loco's had different grates owing to the variance in the firebox size including dimensions. There's a lot of good information in most of the specific class books on sale or were.
AS a fireman, much of our early learning was very much dependant on having good drivers to help you with basic road knowledge & to know where the grades were. In basic training you worked 4 trial trips with an inspector who showed you how to fire, & pointed out various tips as did the driver with you. After those trips if you passed you went out on 80hours, learning, which meant you had to do a minimum of 80 hours of actual in cab fireing experience which was on low level goods, & local passenger working, it primarilly meant working trip & local gods trains & on yard shunters.
Once you got that time in you went back cleaning before progressing to B trials which was on deep sea/long distance working, & qualified for 36, 38cl & other shovel engienes on all main lines out of your depot.
Steam, water, were balancing acts & you learnt by experience & took in every tip you could.
Steam Ejector, I presume the issue is the ejector system as on 3616 the Giesel Oblong Ejector, do a wikin on it.
AS for the fuseable plugs, again this is dependant on the water in boiler which has to stay above the plug lines. Basically it comes back to the amount of water showing in the gauge glasses. If there is water showing in the glass then you are ok. If the water cannot be seem then you need to test if the gauge is faultly. This is done by closing the lever to the side of the glass, & open the drain cock under the gauge glass at the bottom of the fitting.
This drains all water from the glass, & the isolating handle keeps the gauge closed preventing water from the boiler from entering the gauge. Close the drain cock, & then close the isolating cock, this should bring water back into the glass, if nothing happens then there is likely to be a problem.
When water gets low the importance is to get the water level up, even use the two injectors. A crew should never allow the water to get below the bottom level in the gauge which is the bottom of the sight glass. At that point all is ok as with the exception of old 30cl & others, there is at least 2-3 inches between the bottom of the site glass & the bottom level of the water inflow.
One other aspect of the brick arch (& skitz has described it well) is that as engines fitted with brick arches generally have larger fire box & grate areas, drawing the fire & heat along & under the arch, in front of the firebox door, & then over the top of the arch, helps in keeping out cold air from entering through the firehole door. While having the door open helps with certain coals, in the combustion as well as keeps down the smoke level, it is the opposite on other types of coal. Likewise certain steam locomotives were fired with the door totally open all the time & others partially open.