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Tried a couple of things when messing with fluid energy sources on loaders, and none of them function like one would assume. Having only fluid_usage_per_tick and energy_per_item = nil or 0J, loaders will work regardless of if they have fuel and flash a warning if they don't have fuel. Fuel will not be consumed in this configuration. Having a fluid_usage_per_tick and nonzero energy_per_item causes the loader to consume fuel based on fluid_usage_per_tick, but no items will be transferred under any circumstances.
I haven't tested other configurations but I would guess that they don't run into these issues, as for all other setups there is some energy ratio per fluid being considered for energy_per_item.

The loader not working when fluid_usage_per_tick is set with a nonzero energy_per_item makes sense, although perhaps this should result in a startup error instead of weird interactions. I would expect that fluid_usage_per_tick with energy_per_item = nil or 0J would make the loader work, but only when it has fuel, but perhaps that's a bad assumption. If neither of these are in fact bugs, then I would like for a way to have a loader work when it has fluid and only when it has fluid.

A lot of your "assume" lays in the undefined territory where i made arbitrary choices that are easier to implement. For example if energy_per_item == 0, then loader completly skips energy source logic which i is done so that a default loader with no energy source is not punished with energy source logic. Loaders have items transfer decoupled from the energy source logic through a `transfersToPay` variable (visible in the loader tooltip if you have debug tooltips enabled) where each item transfer will increase the transfersToPay variable and later the energy source will be asked to give some energy to reduce that value. This approach was required because item transfers are discrete events while energy source provides energy in a more continous manner and it allowed to have average transfer rates that are smaller than 2 stacks per tick (since there are 2 lanes). Loader only stops transfers when previous transfers were not fully covered in energy consumed.

Given that you are not pointing at any exact behavior that is clearly a bug, i am moving this topic to modding interface requests where we could have a more detailed discussion about what you want to achieve and if its feasible.

I want to have a loader with an energy source (in this case fluid) that consumes power all the time regardless of if it is actually doing work. If possible, I'd like to have the power consumption not affected by how many items it moves. If the base request (consuming power regardless of work) is not possible, then I'd like to be able to specify an amount of fluid to remove per item instead of relying on energy_per_item and the fluid energy source consumption calculations, to avoid needing to add a fuel value to the fluid or modulating it's temperature.