just a few quick points:

- you should probably always have a garrison of 100 men; keep less, and you may get bandits.
- Upkeep for peasant garrisons has never, ever, ever been a problem for me. It's such a small fraction of your total upkeep that it's not workth bothering with it. Normally, by mid-game, you should be churning some 20-70k per year...who cares about 2-3k for peasants ? It's much much worse when the province rebels for some cause (as Doug-Thompson pointed out: disasters, death of influential king, fleet blockade, and you lose half of your empire just because you didn't have 10 or more peasant units in your garrisons...
- There is a way to increase the probability of new units with higher acumen: by having an influential king. When you do, new trained units spawn with higher stats: command, acumen, loyalty (not sure about dread). It's not a big difference: you get, let's say, 3-command generals instead of 1 or 2 stars, or 4-5 acumen instead of 2-3 ones.