well ya brought some interesting questions and observations
maybe i can answer a few of them
1. these engine's don't like to run partly loaded, or running at low loads...
basically it comes down to making heat, alot of folks run with hopper cooling and in order to have enough capacity to cool properly
at full loading there is simply too much cooling capacity at part/low load operation. the engines turn slowly which allows more time for the
heat to be sucked away to the relatively cold coolant, less heat until the engine is not combusting properly and it starts to slobber.
the injection systems play a part in the game as well, the are relatively low pressure injection compared to todays engines. this in itself is not
a problem when operated up to temp and running under load
2. economy plays a significant roll as well, you mention 3 gallon per day vs 1.5 gallon per day...
i guess this comes down to the available fuel to be used, if it is 5 dollar diesel as opposed to "free" waste veggie, it makes a difference
and who knows what the fuel price will be next week much less next year or decade. even the "free" waste oils might very well be legislated out of existance to the diy'er,, who can say for sure. so it would seem prudent to at least recognize economy as being important if not now
certainly down the road.
this also plays in a decision to part load an engine,
the baseline fuel consumption of an engine can be as much as a 1/4 gallon per hour just to turn itself over, add belts and a generator
and it goes up from there, larger engine's can consume even more.
so what happens is the first kwatt generated it the most expensive kwatt, the second splits the baseline with the first, the third splits the baseline with the first and second and so on.
the result being any engine will consume less fuel per kwatt/hr generated if it is working at or near its max capability.
there are tons of other factors that play into this question
while in practice you could use a radiator, electric fan, and t stat to keep the coolant temps up to normal temps to a point
very large engines will still have problems making enough heat to burn as cleanly and efficiently at low loading.
you already have a smaller diesel set, in my opinion that should be the one that carries the load most of the time, and then use
the big guy to carry the heavy loads as needed
hope this helps
bob g