Hey.
Got one question bothering me since I was playing RTW. Never had a chance to ask, so here it is.. (Don't wanna make mess with useless threads, so I'm asking in this one)
How many units do you usually leave in cities? Do you have to increase the number of units as the population grows bigger? How to check if I have enough units in my city?
In bigger cities I usually leave 4 units of infantry militia and 4 of archers, and in smaller 2 units of each.. Sometimes I put better/more units if the city is on neighbour/enemy (especially the ones with weak relationships) borders, so it doesn't draw any attention.
I can't find any specific information about the ratio garrison

opulation.. I don't want to waste money on unnecessary garrison.
P.S Just reading through M2TW guides, if I find anything, I'll delete this question
1 unit, sometimes 0 (when I have troop production that will finish at the end of the turn in that city).
As to your original point, I would rank the the factions like so:
Top Tier: Islamic factions
1 The Turks
2 Egypt
3 The Moors
The Muslim factions are the richest because they have access to completely free troops from turn 1, in order to separate them I then take into account starting position, number and type of cities as well as distance and ease of potential conquests. I do rate Egypt's starting position, trade revenues and profitability higher than that of the Turks, however, the Turks can easily sack Constantinople within the first 5 turns and the money gained from sacking as well as the income from such a huge city will then drive your forward at a great pace. The Moors I do not believe to be close to the other two.
Second Tier: Italian factions plus the HRE
4 Sicily
5 Venice
6 Milan
7 HRE
These factions are chosen because of their proximity to Rome and therefore they are ranked by how quickly they can have a crusade called in order to have free troop upkeep. The Italian factions are remarkably even in their economies with I think Venice taking the slight edge and also being the best in terms of starting position. Added to this is access to the sea, enabling crusading armies to assault and sack a city before sailing to besiege another all in the same turn. HRE appears poor, but if you build a town hall in Bologna and train a diplomat you can have a crusade called by turn 5. HRE is hard to get off the ground but if you can make sure in the early turns to capture 2-3 settlements a turn the money from sacks, building demolition and taxes can turn central europe into a germanic powerhouse very quickly.
Third Tier: The not quite Italian catholics
8 Hungary
9 France
10 Portugal
11 Spain
These factions benefit from being reasonably close to the Pope, but not quite close enough for that lightning quick crusade to get your economy pulsing. Hungary I consider the best of these factions as they are amongst the closest to the Pope as well as being in a prime position to sack Constantinople and being closish to the sea enables good mobility. France is good because the position and territories it starts with are good, it is very close to the Pope and also has good access to the Mediterranean sea. Spain and Portugal are very similar, I think the only real difference is that Portugal will most likely take Zaragoza first and the Spaniards get Valencia and I'd rather have an easy fight for a city than a tougher battle to take a castle.
Fourth Tier: The rest of the catholics
12 England
13 Poland
14 Scotland
15 Denmark
England are the best here because they are the biggest and strongest and they also have a great starting position. Poland edge out Scotland because although they have an inferior starting position, they are bigger and they can get to Constantinople and the holy lands much faster. Denmark is the worst because the Baltic is not as prosperous as the British Isles and other than that they are much the same as Scotland.
Fifth Tier: The Orthodox factions
16 Byzantium
17 Russia
No crusades, no jihad, no hope. Having a great starting position with some tremendous cities under your control is of little consequence when other factions have completely free armies that apparently ride trucks who can recruit some incredibly powerful mercs that the Orthodox factions never have access to, talk about unfair. Sure they can be stinking rich, but they will never be as rich as a human controlled Islamic or Catholic faction. As for Russia, long boring marches through the Steppes to well defended but poor settlements is about all you are gonna get. At least you will be reasonably rich because you won't control enough settlements to spend all your money.
I know there will be those who say I was Russia once and I had loads of money. That may be true, but you'd have had even more money at that stage with almost any other faction.
One last point, this all really depends on what timescale you are looking to become rich over, mine is quite long as my strategy to become rich is to enslave the entire world to work for my bidding, so I think that the richest factions generally coincide with the factions best suited to a quick world conquest.