Rob, I don't think there are any really hard and fast rules re: alternator
capacity vs. mominal load (other than being able to supply any continuous
loads without depleting your battery over time).

You need some reasonable amount of alternator capacity over and above the
nominal aircraft loads if for nothing else, recharging your battery right
after start-up in some time acceptable to you. How much is hard to figure -
depends on what you deem OK and whether or not your engine is a quick
starter (some hot-starts with injected engines I've seen can take a lot out
of a battery - that big sucking sound is the starter motor and the starter
contactor draws a couple of amps too).

If your nominal loads add up to, say 60% of capacity, you've got roughly 40%
to recharge the battery. After it's recharged, you're carrying excess
baggage - you just need to balance how quickly you want the battery
recharged and how much excess baggage you wish to haul.

Of course, not exactly all 40% is excess as you do need the capability to
supply the occasional loads (pitot heat and the like) but even there, the
occasional loads could dip into the battery and the alternator could then
replace it when those loads are off. Not all that good an idea but I've seen
it done on a few airplanes to reduce weight to absolute minimum (Rutan's
maxim regarding weight, "throw it up in the air and if it comes down, it's
too heavy", really took with some folks)!

Sorry I didn't put forth any definitive answers to your questions but I hope
this post will give you more pieces to the puzzle.

Dan Schaefer