I posted this already in the squalor thread, but wanted to post here too since it's a more obvious place to discuss it. Basically, aside from the fact that peasants count 1/2 for PO, the PO system for garrisoning seems very easy to determine: in short, it's entirely based on the number of actual men in the garrison. I'll cite the results that indicate this from the other thread:

Quote Originally Posted by d3nn16
I was asking myself what would be the best unit for garrison :
I'm playing as Venice and after some research I found this (I'm not playing the English version so the translation may not be the same) :

PO---Peasants--Crossbow--Archer-----Italian--Spearman--Crossbow--Mounted
----------------Peasants---Peasants--Militia---Militia------Militia------Militia
5%-----5----------3---------3---------3---------3---------3---------5
10%----8----------5---------5---------4---------4---------5---------7
15%----12---------8---------8---------6---------6---------8
20%----15---------10--------10--------8---------8---------10
25%----20---------13------------------10--------10
30%----------------15-----------------12--------12
35% ---------------17-----------------14---------14
40%----------------19,20--------------16 --------16
45%-----------------------------------18---------18
50%------------------------------------19--------19
As we can clearly see, Italian Militia and Spearman Militia are the best performers, and coincidentally have the largest base unit size. Archer Peasants and Crossbow Militia are in second, again having the same default archer unit size that is a little lower than that of the spear units. Then come the mounted militia performing poorly, as they have a far smaller unit size. Last are the peasants, as they are half a man for garrison purposes and therefore just a hair less effective than mounted militia since 60 / 2 = 30 is what they count as for garrison PO, where cavalry have base unit size 32.

So at least for PO purposes it seems you should garrison the largest unit at your disposal, which is a fact easily missed if you have not specifically tested it or noticed that different units cause different amounts of public order. There of course may be other reasons to garrison archers or cavalry, as they both can play very useful roles in a defensive capacity.

For those of you further interested in in the calculations, it appears that the 20% level represents the least common multiple of units with 40 men, units with 60 men, and units with 75 men (note I'm playing with unit sizes 1.25 times the base values listed in the file, apparently). That is to say, that level is attained exactly by an even number of each unit size, and represents 600 men in the garrison. Then if 600 men equals 20%, we know that 300 equals 10%, and 150 equals 5%. So as it turns out, 2 units of spearmen are exactly enough to give a 5% bonus to garrison PO, which is a very useful thing to know. Could prove very helpful in balancing those cities that are just on the edge, without so much guesswork...