remember him saying he'll use the bleed air to start up the other one,
or something to that effect.

Normally you use APU bleed to start the engines. If the APU is dead,
you can still dispatch by using a ground air cart to start one engine
then cross bleed to start the other(s). Bleed air from the engines is
used to pressurize the cabin.

hadn't known earlier was that pilots actually calculate the extent of
throttle required for takeoff, based on the aircraft loading; I'd
somehow presumed it was always full throttle. On that flight, the cap'n
took off at 90% or so, because there were only 12 souls and almost no
luggage on board

It really has very little to do with load by itself, but runway
available and conditions such as runway altitude, temperature, etc.
Most jet aircraft either use one of two power settings, max or reduced
engine pressure ratio (EPR). Reduced on most aircraft is 10% lower EPR
than max. Some large engines use the N1 (fan speed) for power
indication, so I would imagine they set a reduced N1. The reduced
power setting is used to save wear and tear on the engines, if there
is enough available runway for the conditions. Most takeoffs are at a
reduced power even with a full load. One operator's certificate I am
aware of require them to log at least one full power takeoff per month
to ensure the engines can make full power when required.

Most modern aircraft are capable of calculating the required takeoff
distance based on the sensed weight, temperature, etc. and will adjust
the autothrottles to obtain max or reduced EPR or N1 as selected by
the crew.

Each engine is an individual and will be slightly different from any
other in fuel flow, temperature, or rpm at any particular power
setting. Most common is to match N1 to reduce the out-of-sync noise
in the cabin. Everything else will be a slight mismatch.

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